How geometry, stopwatches, and Einstein's theories work together to make GPS possible.

If you're like me, you might be entirely dependent on GPS to navigate the world. At some point, you may have caught yourself wondering during those panicked moments when an exit is coming up and your phone is recalibrating: how does my phone even know where I am?

The answer is in some ways simpler than you'd expect, and in other ways more complex. GPS is fundamentally a translation tool: it converts time into distance. A satellite sends a signal, your phone catches it, and the delay between those two events tells the phone exactly how far away the satellite is. Everything else is about making that measurement precise enough to be useful: accounting for bad clocks, satellite geometry, and eventually, Einstein's theories.

The Ruler

Every GPS measurement starts with a stopwatch. A satellite broadcasts a signal at the speed of light. Your phone receives it and checks how long the trip took. Multiply the travel time by the speed of light, and you get the distance.

One Satellite, One Ring

Measuring a single satellite gives you a distance, but not a direction. If a signal takes to reach your phone, you are roughly from the satellite. If you took every point at that distance from the satellite, you would get a ring on the surface of the Earth (technically an oblate spheroid, but effectively a ring for our purposes). One satellite tells us we're somewhere on that ring, but it can't tell us where exactly.

Not to scale

Three Satellites, One Point

One ring isn't enough since you could be anywhere along it. A second satellite produces a second ring which crosses the first one at exactly two points. A third satellite produces a third ring, which passes through only one of those two points.

This process is called trilateration. Each satellite gives you one equation:

is the known position of satellite , and is the measured distance. We can solve for three unknowns with three equations.

Not to scale

The Clock Problem

There's a problem with the math above: it assumes your phone knows the travel time perfectly.

Each GPS satellite carries an incredibly precise atomic clock, accurate to about . Your phone has a much cheaper quartz crystal oscillator that can naturally drift by microseconds (thousands of nanoseconds). Since of clock error produces of position error, even of drift puts you off. Without accounting for this, GPS would become pretty useless pretty quickly!

The fix is to add another satellite.

In simple terms: there is only one specific clock correction possible where all four spheres intersect at a single, perfect point. The 4th satellite gives the receiver enough information to find it. Once it does, it corrects every distance measurement at once, and the previously fuzzy answer snaps into focus. Conceptually, you can think about the system doing some math to figure out how to make the new red ring below perfectly intersect with the other three rings.

Not to scale

The Relativity Tax

Even with four satellites and a solved clock, we're not quite done yet.

To understand why, we have to think of time itself as a clock that can be sped up or slowed down by its surroundings. GPS has to account for two specific distortions:

• Special Relativity (speed): Einstein discovered that the faster an object moves, the slower time passes for it. GPS satellites move at roughly , so their clocks lose about per day compared to ours.
• General Relativity (gravity): Gravity also warps time. The further you are from a massive object like Earth, the faster time ticks. The satellites orbit at altitude in weaker gravity, so their clocks gain about per day.

These two effects don't cancel out. The gravity gain is much stronger than the speed loss.

Without a correction, the satellite clocks would run ahead of ground clocks every day. Because light travels every microsecond, that small offset would cause your position to drift by roughly every 24 hours.

Engineers bake this correction into the hardware. The satellite clocks are built to tick slightly too slow on the ground, at instead of the nominal . Once in orbit, the combination of weaker gravity and orbital speed makes them tick at exactly the correct rate.

Not to scale

Without these corrections, GPS would become unusable within hours. The fact that your phone can pinpoint your location to within a few meters is, in addition to being a modern miracle, a quiet and continuous proof that Einstein was right.

A Joint Effort

In practice, your phone doesn't stop at four satellites. Modern receivers typically lock onto 8 to 12 at once, sometimes more. The extra signals don't change the core math, but they let the receiver average out errors and pick the best satellite geometry. More satellites means sharper intersections and a more stable fix.

And it's not just the American GPS constellation. Russia operates GLONASS, the EU has Galileo, and China has BeiDou. Your phone can listen to all of them simultaneously. That means over 100 atomic stopwatches orbiting overhead, built by different countries, all working together to tell you where you are.

Distances and Earth's size are to scale. Showing 147 live GNSS satellites via CelesTrak (data refreshed within last hour).

Satellite placement also matters. If the satellites are clumped together in one part of the sky, their rings intersect at very shallow angles. This creates a wide, blurry area of uncertainty around the true position. GPS engineers call this Geometric Dilution of Precision (GDOP). Good geometry means satellites spread across the sky, so that their rings cross at sharp angles and produce a tight, high-confidence intersection point. Your phone's GPS chip automatically selects the best combination of visible satellites to minimize GDOP.

In cities, GPS signals can bounce off buildings before reaching your phone. This makes the stopwatch think you are further away than you actually are, because the signal took a longer path. This is called multipath error, and it's the main reason GPS gets less accurate in dense urban areas. Modern receivers use multiple techniques to detect and filter out these reflected signals, but it remains one of the hardest problems in GPS.

With all that said, I find it amazing that your phone can pinpoint your location to within a few meters using nothing more than the time it takes light to travel from a few satellites tens of thousands of kilometers away.

If you want to go much deeper, Bartosz Ciechanowski's interactive explainer on GPS is the gold standard. It covers signal modulation, orbital mechanics, and receiver architecture in far more detail than we do here.

Thank you!

If you like this type of content, you can follow me on BlueSky. If you wanted to support me further, buying me a coffee would be much appreciated. It helps us keep the lights on and the servers running! ☕

Today we are excited to announce the availability of TypeScript 6.0!

If you are not familiar with TypeScript, it’s a language that builds on JavaScript by adding syntax for types, which enables type-checking to catch errors, and provide rich editor tooling. You can learn more about TypeScript and how to get started on the TypeScript website.

But if you’re already familiar with the language, you can get TypeScript 6.0 through npm with the following command:

npm install -D typescript

TypeScript 6.0 is a unique release in that we intend for it to be the last release based on the current JavaScript codebase. As announced last year (with recent updates here), we are working on a new codebase for the TypeScript compiler and language service written in Go that takes advantage of the speed of native code and shared-memory multi-threading. That new codebase will be the foundation of TypeScript 7.0 and beyond.

TypeScript 6.0 acts as the bridge between TypeScript 5.9 and 7.0. As such, most changes in TypeScript 6.0 are meant to help align and prepare for adopting TypeScript 7.0. It may seem surprising to say, but TypeScript 7.0 is actually extremely close to completion. You can try it out in Visual Studio Code or install it from npm. In fact, if you’re able to adopt TypeScript 6.0, we encourage you to try out the native previews of TypeScript 7.0.

With that said, there are some new features and improvements in TypeScript 6.0s that are not just about alignment. Let’s take a look at some of the highlights of this release, followed by a more detailed look at what’s changing for 7.0 and how to prepare for it.

What’s New Since the Beta and RC?

Since TypeScript 6.0 beta, we have made a few noteworthy changes – mostly to align with the behavior of TypeScript 7.0.

One adjustment is in type-checking for function expressions in generic calls, especially those occurring in generic JSX expressions (see this pull request). This will typically catch more bugs in existing code, though you may find that some generic calls may need an explicit type argument.

We have also extended our deprecation of import assertion syntax (i.e. import ... assert {...}) to import() calls like import(..., { assert: {...}})

Finally, we have updated the DOM types to reflect the latest web standards, including some adjustments to the Temporal APIs as well.

Less Context-Sensitivity on this-less Functions

When parameters don’t have explicit types written out, TypeScript can usually infer them based on an expected type, or even through other arguments in the same function call.

declare function callIt<T>(obj: {
    produce: (x: number) => T,
    consume: (y: T) => void,
}): void;

// Works, no issues.
callIt({
    produce: (x: number) => x * 2,
    consume: y => y.toFixed(),
});

// Works, no issues even though the order of the properties is flipped.
callIt({
    consume: y => y.toFixed(),
    produce: (x: number) => x * 2,
});

Here, TypeScript can infer the type of y in the consume function based on the inferred T from the produce function, regardless of the order of the properties. But what about if these functions were written using method syntax instead of arrow function syntax?

declare function callIt<T>(obj: {
    produce: (x: number) => T,
    consume: (y: T) => void,
}): void;

// Works fine, `x` is inferred to be a number.
callIt({
    produce(x: number) { return x * 2; },
    consume(y) { return y.toFixed(); },
});

callIt({
    consume(y) { return y.toFixed(); },
    //                  ~
    // error: 'y' is of type 'unknown'.

    produce(x: number) { return x * 2; },
});

Strangely enough, the second call to callIt results in an error because TypeScript is not able to infer the type of y in the consume method. What’s happening here is that when TypeScript is trying to find candidates for T, it will first skip over functions whose parameters don’t have explicit types. It does this because certain functions may need the inferred type of T to be correctly checked – in our case, we need to know the type of T to analyze our consume function.

These functions are called contextually sensitive functions – basically, functions that have parameters without explicit types. Eventually the type system will need to figure out types for these parameters – but this is a bit at odds with how inference works in generic functions because the two "pull" on types in different directions.

function callFunc<T>(callback: (x: T) => void, value: T) {
    return callback(value);
}

callFunc(x => x.toFixed(), 42);
//       ^
// We need to figure out the type of `x` here,
// but we also need to figure out the type of `T` to check the callback.

To solve this, TypeScript skips over contextually sensitive functions during type argument inference, and instead checks and infers from other arguments first. If skipping over contextually sensitive functions doesn’t work, inference just continues across any unchecked arguments, going left-to-right in the argument list. In the example immediately above, TypeScript will skip over the callback during inference for T, but will then look at the second argument, 42, and infer that T is number. Then, when it comes back to check the callback, it will have a contextual type of (x: number) => void, which allows it to infer that x is a number as well.

So what’s going on in our earlier examples?

// Arrow syntax - no errors.
callIt({
    consume: y => y.toFixed(),
    produce: (x: number) => x * 2,
});

// Method syntax - errors!
callIt({
    consume(y) { return y.toFixed(); },
    //                  ~
    // error: 'y' is of type 'unknown'.

    produce(x: number) { return x * 2; },
});

In both examples, produce is assigned a function with an explicitly-typed x parameter. Shouldn’t they be checked identically?

The issue is subtle: most functions (like the ones using method syntax) have an implicit this parameter, but arrow functions do not. Any usage of this could require "pulling" on the type of T – for example, knowing the type of the containing object literal could in turn require the type of consume, which uses T.

But we’re not using this! Sure, the function might have a this value at runtime, but it’s never used!

TypeScript 6.0 takes this into account when it decides if a function is contextually sensitive or not. If this is never actually used in a function, then it is not considered contextually sensitive. That means these functions will be seen as higher-priority when it comes to type inference, and all of our examples above now work!

This change was provided thanks to the work of Mateusz Burzyński.

Subpath Imports Starting with #/

When Node.js added support for modules, it added a feature called "subpath imports". This is basically a field called imports which allows packages to create internal aliases for modules within their package.

{
    "name": "my-package",
    "type": "module",
    "imports": {
        "#root/*": "./dist/*"
    }
}

This allows modules in my-package to import from paths starting with #root/

import * as utils from "#root/utils.js";

instead of using a relative path like the following.

import * as utils from "../../utils.js";

One minor annoyance with this feature has been that developers always had to write something after the # when specifying a subpath import. Here, we used root, but it is a bit useless since there is no directory we’re mapping over other than ./dist/

Developers who have used bundlers are also accustomed to using path-mapping to avoid long relative paths. A familiar convention with bundlers has been to use a simple @/ as the prefix. Unfortunately, subpath imports could not start with #/ at all, leading to a lot of confusion for developers trying to adopt them in their projects.

But more recently, Node.js added support for subpath imports starting with #/. This allows packages to use a simple #/ prefix for their subpath imports without needing to add an extra segment.

{
    "name": "my-package",
    "type": "module",
    "imports": {
        "#/*": "./dist/*"
    }
}

This is supported in newer Node.js 20 releases, and so TypeScript now supports it under the options nodenext and bundler for the --moduleResolution setting.

This work was done thanks to magic-akari, and the implementing pull request can be found here.

Combining --moduleResolution bundler with --module commonjs

TypeScript’s --moduleResolution bundler setting was previously only allowed to be used with --module esnext or --module preserve; however, with the deprecation of --moduleResolution node (a.k.a. --moduleResolution node10), this new combination is often the most suitable upgrade path for many projects.

Projects will often want to instead plan out a migration towards either

• --module preserve and --moduleResolution bundler
• --module nodenext

depending on your project type (e.g. bundled web app, Bun app, or Node.js app).

More information can be found at this implementing pull request.

The --stableTypeOrdering Flag

As part of our ongoing work on TypeScript’s native port, we’ve introduced a new flag called --stableTypeOrdering intended to assist with 6.0-to-7.0 migrations.

Today, TypeScript assigns type IDs (internal tracking numbers) to types in the order they are encountered, and uses these IDs to sort union types in a consistent manner. A similar process occurs for properties. As a result, the order in which things are declared in a program can have possibly surprising effects on things like declaration emit.

For example, consider the declaration emit from this file:

// Input: some-file.ts
export function foo(condition: boolean) {
    return condition ? 100 : 500;
}

// Output: some-file.d.ts
export declare function foo(condition: boolean): 100 | 500;
//                                               ^^^^^^^^^
//             Note the order of this union: 100, then 500.

If we add an unrelated const above foo, the declaration emit changes:

// Input: some-file.ts
const x = 500;
export function foo(condition: boolean) {
    return condition ? 100 : 500;
}

// Output: some-file.d.ts
export declare function foo(condition: boolean): 500 | 100;
//                                               ^^^^^^^^^
//                           Note the change in order here.

This happens because the literal type 500 gets a lower type ID than 100 because it was processed first when analyzing the const x declaration. In very rare cases this change in ordering can even cause errors to appear or disappear based on program processing order, but in general, the main place you might notice this ordering is in the emitted declaration files, or in the way types are displayed in your editor.

One of the major architectural improvements in TypeScript 7 is parallel type checking, which dramatically improves overall check time. However, parallelism introduces a challenge: when different type-checkers visit nodes, types, and symbols in different orders, the internal IDs assigned to these constructs become non-deterministic. This in turn leads to confusing non-deterministic output, where two files with identical contents in the same program can produce different declaration files, or even calculate different errors when analyzing the same file. To fix this, TypeScript 7.0 sorts its internal objects (e.g. types and symbols) according to a deterministic algorithm based on the content of the object. This ensures that all checkers encounter the same object order regardless of how and when they were created. As a consequence, in the given example, TypeScript 7 will always print 100 | 500, removing the ordering instability entirely.

This means that TypeScript 6 and 7 can and do sometimes display different ordering. While these ordering changes are almost always benign, if you’re comparing compiler outputs between runs (for example, checking emitted declaration files in 6.0 vs 7.0), these different orderings can produce a lot of noise that makes it difficult to assess correctness. Occasionally though, you may witness a change in ordering that causes a type error to appear or disappear, which can be even more confusing.

To help with this situation, in 6.0, you can specify the new --stableTypeOrdering flag. This makes 6.0’s type ordering behavior match 7.0’s, reducing the number of differences between the two codebases. Note that we don’t necessarily encourage using this flag all the time as it can add a substantial slowdown to type-checking (up to 25% depending on codebase).

If you encounter a type error using --stableTypeOrdering, this is typically due to inference differences. The previous inference without --stableTypeOrdering happened to work based on the current ordering of types in your program. To help with this, you’ll often benefit from providing an explicit type somewhere. Often, this will be a type argument

- someFunctionCall(/*...*/);
+ someFunctionCall<SomeExplicitType>(/*...*/);

or a variable annotation for an argument you intend to pass into a call.

- const someVariable = { /*... some complex object ...*/ };
+ const someVariable: SomeExplicitType = { /*... some complex object ...*/ };

someFunctionCall(someVariable);

Note that this flag is only intended to help diagnose differences between 6.0 and 7.0 – it is not intended to be used as a long-term feature

See more at this pull-request.

es2025 option for target and lib

TypeScript 6.0 adds support for the es2025 option for both target and lib. While there are no new JavaScript language features in ES2025, this new target adds new types for built-in APIs (e.g. RegExp.escape), and moves a few declarations from esnext into es2025 (e.g. Promise.try, Iterator methods, and Set methods). Work to enable the new target was contributed thanks to Kenta Moriuchi.

New Types for Temporal

The long-awaited Temporal proposal has reached stage 4 and will be part of a future ECMAScript standard. TypeScript 6.0 now includes built-in types for the Temporal API, so you can start using it in your TypeScript code today via --target esnext or "lib": ["esnext"] (or the more-granular esnext.temporal).

let yesterday = Temporal.Now.instant().subtract({
    hours: 24,
});

let tomorrow = Temporal.Now.instant().add({
    hours: 24,
});

console.log(`Yesterday: ${yesterday}`);
console.log(`Tomorrow: ${tomorrow}`);

Temporal is already usable in several runtimes, and with stage 4 status it is now officially part of the JavaScript language. Documentation on the Temporal APIs is available on MDN.

This work was contributed thanks to GitHub user Renegade334.

New Types for "upsert" Methods (a.k.a. getOrInsert)

A common pattern with Maps is to check if a key exists, and if not, set and fetch a default value.

function processOptions(compilerOptions: Map<string, unknown>) {
    let strictValue: unknown;
    if (compilerOptions.has("strict")) {
        strictValue = compilerOptions.get("strict");
    }
    else {
        strictValue = true;
        compilerOptions.set("strict", strictValue);
    }
    // ...
}

This pattern can be tedious. ECMAScript’s "upsert" proposal recently reached stage 4, and introduces 2 new methods on Map and WeakMap:

• getOrInsert
• getOrInsertComputed

These methods have been added to the esnext lib so that you can start using them immediately in TypeScript 6.0.

With getOrInsert, we can replace our code above with the following:

function processOptions(compilerOptions: Map<string, unknown>) {
    let strictValue = compilerOptions.getOrInsert("strict", true);
    // ...
}

getOrInsertComputed works similarly, but is for cases where the default value may be expensive to compute (e.g. requires lots of computations, allocations, or does long-running synchronous I/O). Instead, it takes a callback that will only be called if the key is not already present.

someMap.getOrInsertComputed("someKey", () => {
    return computeSomeExpensiveValue(/*...*/);
});

This callback is also given the key as an argument, which can be useful for cases where the default value is based on the key.

someMap.getOrInsertComputed(someKey, computeSomeExpensiveDefaultValue);

function computeSomeExpensiveValue(key: string) {
    // ...
}

This update was contributed thanks to GitHub user Renegade334.

RegExp.escape

When constructing some literal string to match within a regular expression, it is important to escape special regular expression characters like *, +, ?, (, ), etc. The RegExp Escaping ECMAScript proposal has reached stage 4, and introduces a new RegExp.escape function that takes care of this for you.

function matchWholeWord(word: string, text: string) {
    const escapedWord = RegExp.escape(word);
    const regex = new RegExp(`\\b${escapedWord}\\b`, "g");
    return text.match(regex);
}

RegExp.escape is available in the es2025 lib, so you can start using it in TypeScript 6.0 today.

This work was contributed thanks Kenta Moriuchi.

The dom lib Now Contains dom.iterable and dom.asynciterable

TypeScript’s lib option allows you to specify which global declarations your target runtime has. One option is dom to represent web environments (i.e. browsers, who implement the DOM APIs). Previously, the DOM APIs were partially split out into dom.iterable and dom.asynciterable for environments that didn’t support Iterables and AsyncIterables. This meant that you had to explicitly add dom.iterable to use iteration methods on DOM collections like NodeList or HTMLCollection.

In TypeScript 6.0, the contents of lib.dom.iterable.d.ts and lib.dom.asynciterable.d.ts are fully included in lib.dom.d.ts. You can still reference dom.iterable and dom.asynciterable in your configuration file’s "lib" array, but they are now just empty files.

// Before TypeScript 6.0, this required "lib": ["dom", "dom.iterable"]
// Now it works with just "lib": ["dom"]
for (const element of document.querySelectorAll("div")) {
    console.log(element.textContent);
}

This is a quality-of-life improvement that eliminates a common point of confusion, since no major modern browser lacks these capabilities. If you were already including both dom and dom.iterable, you can now simplify to just dom.

See more at this issue and its corresponding pull request.

Breaking Changes and Deprecations in TypeScript 6.0

TypeScript 6.0 arrives as a significant transition release, designed to prepare developers for TypeScript 7.0, the upcoming native port of the TypeScript compiler. While TypeScript 6.0 maintains full compatibility with your existing TypeScript knowledge and continues to be API compatible with TypeScript 5.9, this release introduces a number of breaking changes and deprecations that reflect the evolving JavaScript ecosystem and set the stage for TypeScript 7.0.

In the two years since TypeScript 5.0, we’ve seen ongoing shifts in how developers write and ship JavaScript:

• Virtually every runtime environment is now "evergreen". True legacy environments (ES5) are vanishingly rare.
• Bundlers and ESM have become the most common module targets for new projects, though CommonJS remains a major target. AMD and other in-browser userland module systems are much rarer than they were in 2012.
• Almost all packages can be consumed through some module system. UMD packages still exist, but virtually no new code is available only as a global variable.
• tsconfig.json is nearly universal as a configuration mechanism.
• Appetite for "stricter" typing continues to grow.
• TypeScript build performance is top of mind. Despite the gains of TypeScript 7, performance must always remain a key goal, and options which can’t be supported in a performant way need to be more strongly justified.

So TypeScript 6.0 and 7.0 are designed with these realities in mind. For TypeScript 6.0, these deprecations can be ignored by setting "ignoreDeprecations": "6.0" in your tsconfig; however, note that TypeScript 7.0 will not support any of these deprecated options.

Some necessary adjustments can be automatically performed with a codemod or tool. For example, the experimental ts5to6 tool can automatically adjust baseUrl and rootDir across your codebase.

Up-Front Adjustments

We’ll cover specific adjustments below, but we have to note that some deprecations and behavior changes do not necessarily have an error message that directly points to the underlying issue. So we’ll note up-front that many projects will need to do at least one of the following:

• Set the "types" array in tsconfig, typically to "types": ["node"].

  "types": ["*"] will restore the 5.9 behavior, but we recommend using an explicit array to improve build performance and predictability.

  You’ll typically know this is the issue if you see a lot of type errors related to missing identifiers or unresolved built-in modules.

• Set "rootDir": "./src" if you were previously relying on this being inferred

  You’ll often know this is the issue if you see files being written to ./dist/src/index.js instead of ./dist/index.js.

Simple Default Changes

Several compiler options now have updated default values that better reflect modern development practices.

• strict is now true by default: The appetite for stricter typing continues to grow, and we’ve found that most new projects want strict mode enabled. If you were already using "strict": true, nothing changes for you. If you were relying on the previous default of false, you’ll need to explicitly set "strict": false in your tsconfig.json.

• module defaults to esnext: Similarly, the new default module is esnext, acknowledging that ESM is now the dominant module format.

• target defaults to current-year ES version: The new default target is the most recent supported ECMAScript spec version (effectively a floating target). Right now, that target is es2025. This reflects the reality that most developers are shipping to evergreen runtimes and don’t need to transpile down to older ECMAScript versions.

• noUncheckedSideEffectImports is now true by default: This helps catch issues with typos in side-effect-only imports.

• libReplacement is now false by default: This flag previously incurred a large number of failed module resolutions for every run, which in turn increased the number of locations we needed to watch under --watch and editor scenarios. In a new project, libReplacement never does anything until other explicit configuration takes place, so it makes sense to turn this off by default for the sake of better performance by default.

If these new defaults break your project, you can specify the previous values explicitly in your tsconfig.json.

rootDir now defaults to .

rootDir controls the directory structure of your output files relative to the output directory. Previously, if you did not specify a rootDir, it was inferred based on the common directory of all non-declaration input files. But this often meant that it was impossible to know if a file belonged to a project without trying to load and parse that project. It also meant that TypeScript had to spend more time inferring that common source directory by analyzing every file path in the program.

In TypeScript 6.0, the default rootDir will always be the directory containing the tsconfig.json file. rootDir will only be inferred when using tsc from the command line without a tsconfig.json file.

If you have source files any level deeper than your tsconfig.json directory and were relying on TypeScript to infer a common root directory for source files, you’ll need to explicitly set rootDir:

  {
      "compilerOptions": {
          // ...
+         "rootDir": "./src"
      },
      "include": ["./src"]
  }

Likewise, if your tsconfig.json referenced files outside of the containing tsconfig.json, you would need to adjust your rootDir to include those files.

  {
      "compilerOptions": {
          // ...
+         "rootDir": "../src"
      },
      "include": ["../src/**/*.tests.ts"]
  }

See more at the discussion here and the implementation here.

types now defaults to []

In a tsconfig.json, the types field of compilerOptions specifies a list of package names to be included in the global scope during compilation. Typically, packages in node_modules are automatically included via imports in your source code; but for convenience, TypeScript would also include all packages in node_modules/@types by default, so that you can get global declarations like process or the "fs" module from @types/node, or describe and it from @types/jest, without needing to import them directly.

In a sense, the types value previously defaulted to "enumerate everything in node_modules/@types". This can be very expensive, as a normal repository setup these days might transitively pull in hundreds of @types packages, especially in multi-project workspaces with flattened node_modules. Modern projects almost always need only @types/node, @types/jest, or a handful of other common global-affecting packages.

In TypeScript 6.0, the default types value will be [] (an empty array). This change prevents projects from unintentionally pulling in hundreds or even thousands of unneeded declaration files at build time. Many projects we’ve looked at have improved their build time anywhere from 20-50% just by setting types appropriately.

This will affect many projects. You will likely need to add "types": ["node"] or a few others:

  {
      "compilerOptions": {
          // Explicitly list the @types packages you need
+         "types": ["node", "jest"]
      }
  }

You can also specify a * entry to re-enable the old enumeration behavior:

  {
      "compilerOptions": {
          // Load ALL the types - the default from TypeScript 5.9 and before.
+         "types": ["*"]
      }
  }

If you end up with new error messages like the following:

Cannot find module '...' or its corresponding type declarations.
Cannot find name 'fs'. Do you need to install type definitions for node? Try `npm i --save-dev @types/node` and then add 'node' to the types field in your tsconfig.
Cannot find name 'path'. Do you need to install type definitions for node? Try `npm i --save-dev @types/node` and then add 'node' to the types field in your tsconfig.
Cannot find name 'process'. Do you need to install type definitions for node? Try `npm i --save-dev @types/node` and then add 'node' to the types field in your tsconfig.
Cannot find name 'Bun'. Do you need to install type definitions for Bun? Try `npm i --save-dev @types/bun` and then add 'bun' to the types field in your tsconfig.
Cannot find name 'describe'. Do you need to install type definitions for a test runner? Try `npm i --save-dev @types/jest` or `npm i --save-dev @types/mocha` and then add 'jest' or 'mocha' to the types field in your tsconfig.

it’s likely that you need to add some entries to your types field.

See more at the proposal here along with the implementing pull request here.

Deprecated: target: es5

The ECMAScript 5 target was important for a long time to support legacy browsers; but its successor, ECMAScript 2015 (ES6), was released over a decade ago, and all modern browsers have supported it for many years. With Internet Explorer’s retirement, and the universality of evergreen browsers, there are very few use cases for ES5 output today.

TypeScript’s lowest target will now be ES2015, and the target: es5 option is deprecated. If you were using target: es5, you’ll need to migrate to a newer target or use an external compiler. If you still need ES5 output, we recommend using an external compiler to either directly compile your TypeScript source, or to post-process TypeScript’s outputs.

See more about this deprecation here along with its implementing pull request.

Deprecated: --downlevelIteration

--downlevelIteration only has effects on ES5 emit, and since --target es5 has been deprecated, --downlevelIteration no longer serves a purpose.

Subtly, using --downlevelIteration false with --target es2015 did not error in TypeScript 5.9 and earlier, even though it had no effect. In TypeScript 6.0, setting --downlevelIteration at all will lead to a deprecation error.

See the implementation here.

Deprecated: --moduleResolution node (a.k.a. --moduleResolution node10)

--moduleResolution node encoded a specific version of Node.js’s module resolution algorithm that most-accurately reflected the behavior of Node.js 10. Unfortunately, this target (and its name) ignores many updates to Node.js’s resolution algorithm that have occurred since then, and it is no longer a good representation of the behavior of modern Node.js versions.

In TypeScript 6.0, --moduleResolution node (specifically, --moduleResolution node10) is deprecated. Users who were using --moduleResolution node should usually migrate to --moduleResolution nodenext if they plan on targeting Node.js directly, or --moduleResolution bundler if they plan on using a bundler or Bun.

See more at this issue and its corresponding pull request.

Deprecated: amd, umd, and systemjs values of module

The following flag values are no longer supported

• --module amd
• --module umd
• --module systemjs
• --module none

AMD, UMD, and SystemJS were important during the early days of JavaScript modules when browsers lacked native module support. The semantics of "none" were never well-defined and often led to confusion. Today, ESM is universally supported in browsers and Node.js, and both import maps and bundlers have become favored ways for filling in the gaps. If you’re still targeting these module systems, consider migrating to an appropriate ECMAScript module-emitting target, adopt a bundler or different compiler, or stay on TypeScript 5.x until you can migrate.

This also implies dropped support for the amd-module directive, which will no longer have any effect.

See more at the proposal issue along with the implementing pull request.

Deprecated: --baseUrl

The baseUrl option is most-commonly used in conjunction with paths, and is typically used as a prefix for every value in paths. Unfortunately, baseUrl is also considered a look-up root for module resolution.

For example, given the following tsconfig.json

{
  "compilerOptions": {
    // ...
    "baseUrl": "./src",
    "paths": {
      "@app/*": ["app/*"],
      "@lib/*": ["lib/*"]
    }
  }
}

and an import like

import * as someModule from "someModule.js";

TypeScript will probably resolve this to src/someModule.js, even if the developer only intended to add mappings for modules starting with @app/ and @lib/.

In the best case, this also often leads to "worse-looking" paths that bundlers would ignore; but it often meant that that many import paths that would never have worked at runtime are considered "just fine" by TypeScript.

path mappings have not required specifying baseUrl for a long time, and in practice, most projects that use baseUrl only use it as a prefix for their paths entries. In TypeScript 6.0, baseUrl is deprecated and will no longer be considered a look-up root for module resolution.

Developers who used baseUrl as a prefix for path-mapping entries can simply remove baseUrl and add the prefix to their paths entries:

  {
    "compilerOptions": {
      // ...
-     "baseUrl": "./src",
      "paths": {
-       "@app/*": ["app/*"],
-       "@lib/*": ["lib/*"]
+       "@app/*": ["./src/app/*"],
+       "@lib/*": ["./src/lib/*"]
      }
    }
  }

Developers who actually did use baseUrl as a look-up root can also add an explicit path mapping to preserve the old behavior:

{
  "compilerOptions": {
    // ...
    "paths": {
      // A new catch-all that replaces the baseUrl:
      "*": ["./src/*"],

      // Every other path now has an explicit common prefix:
      "@app/*": ["./src/app/*"],
      "@lib/*": ["./src/lib/*"],
    }
  }
}

However, this is extremely rare. We recommend most developers simply remove baseUrl and add the appropriate prefixes to their paths entries.

See more at this issue and the corresponding pull request.

Deprecated: --moduleResolution classic

The moduleResolution: classic setting has been removed. The classic resolution strategy was TypeScript’s original module resolution algorithm, and predates Node.js’s resolution algorithm becoming a de facto standard. Today, all practical use cases are served by nodenext or bundler. If you were using classic, migrate to one of these modern resolution strategies.

See more at this issue and the implementing pull request.

Deprecated: --esModuleInterop false and --allowSyntheticDefaultImports false

The following settings can no longer be set to false:

• esModuleInterop
• allowSyntheticDefaultImports

esModuleInterop and allowSyntheticDefaultImports were originally opt-in to avoid breaking existing projects. However, the behavior they enable has been the recommended default for years. Setting them to false often led to subtle runtime issues when consuming CommonJS modules from ESM. In TypeScript 6.0, the safer interop behavior is always enabled.

If you have imports that rely on the old behavior, you may need to adjust them:

// Before (with esModuleInterop: false)
import * as express from "express";

// After (with esModuleInterop always enabled)
import express from "express";

See more at this issue and its implementing pull request.

Deprecated: --alwaysStrict false

The alwaysStrict flag refers to inference and emit of the "use strict"; directive. In TypeScript 6.0, all code will be assumed to be in JavaScript strict mode, which is a set of JS semantics that most-noticeably affects syntactic corner cases around reserved words. If you have "sloppy mode" code that uses reserved words like await, static, private, or public as regular identifiers, you’ll need to rename them. If you relied on subtle semantics around the meaning of this in non-strict code, you may need to adjust your code as well.

See more at this issue and its corresponding pull request.

Deprecated: outFile

The --outFile option has been removed from TypeScript 6.0. This option was originally designed to concatenate multiple input files into a single output file. However, external bundlers like Webpack, Rollup, esbuild, Vite, Parcel, and others now do this job faster, better, and with far more configurability. Removing this option simplifies the implementation and allows us to focus on what TypeScript does best: type-checking and declaration emit. If you’re currently using --outFile, you’ll need to migrate to an external bundler. Most modern bundlers have excellent TypeScript support out of the box.

Deprecated: legacy module Syntax for namespaces

Early versions of TypeScript used the module keyword to declare namespaces:

//  Deprecated syntax - now an error
module Foo {
    export const bar = 10;
}

This syntax was later aliased to the modern preferred form using the namespace keyword:

//  The correct syntax
namespace Foo {
    export const bar = 10;
}

When namespace was introduced, the module syntax was simply discouraged. A few years ago, the TypeScript language service started marking the keyword as deprecated, suggesting namespace in its place.

In TypeScript 6.0, using module where namespace is expected is now a hard deprecation. This change is necessary because module blocks are a potential ECMAScript proposal that would conflict with the legacy TypeScript syntax.

The ambient module declaration form remains fully supported:

//  Still works perfectly
declare module "some-module" {
    export function doSomething(): void;
}

See this issue and its corresponding pull request for more details.

Deprecated: asserts Keyword on Imports

The asserts keyword was proposed to the JavaScript language via the import assertions proposal; however, the proposal eventually morphed into the import attributes proposal, which uses the with keyword instead of asserts.

Thus, the asserts syntax is now deprecated in TypeScript 6.0, and using it will lead to an error:

//  Deprecated syntax - now an error.
import blob from "./blahb.json" asserts { type: "json" }
//                              ~~~~~~~
// error: Import assertions have been replaced by import attributes. Use 'with' instead of 'asserts'.

Instead, use the with syntax for import attributes:

//  Works with the new import attributes syntax.
import blob from "./blahb.json" with { type: "json" }

See more at this issue and its corresponding pull request.

Deprecated: no-default-lib Directives

The /// <reference no-default-lib="true"/> directive has been largely misunderstood and misused. In TypeScript 6.0, this directive is no longer supported. If you were using it, consider using --noLib or --libReplacement instead.

See more here and at the corresponding pull request.

Specifying Command-Line Files When tsconfig.json Exists is Now an Error

Currently, if you run tsc foo.ts in a folder where a tsconfig.json exists, the config file is completely ignored. This was often very confusing if you expected checking and emit options to apply to the input file.

In TypeScript 6.0, if you run tsc with file arguments in a directory containing a tsconfig.json, an error will be issued to make this behavior explicit:

error TS5112: tsconfig.json is present but will not be loaded if files are specified on commandline. Use '--ignoreConfig' to skip this error.

If it is the case that you wanted to ignore the tsconfig.json and just compile foo.ts with TypeScript’s defaults, you can use the new --ignoreConfig flag.

tsc --ignoreConfig foo.ts

See more at this issue and its corresponding pull request.

Preparing for TypeScript 7.0

TypeScript 6.0 is designed as a transition release. While options deprecated in TypeScript 6.0 will continue to work without errors when "ignoreDeprecations": "6.0" is set, those options will be removed entirely in TypeScript 7.0 (the native TypeScript port). If you’re seeing deprecation warnings after upgrading to TypeScript 6.0, we strongly recommend addressing them before adopting TypeScript 7.0 (or trying native previews) in your project.

What’s Next?

Now that TypeScript 6.0 is available on npm, the team will be focused on bringing TypeScript 7.0 to stability. This is much closer than it might sound: we expect a release within a few months, and we are already seeing broad adoption inside and outside of Microsoft on extremely large codebases. So we encourage teams to try out nightly builds of TypeScript 7.0’s native previews on npm along with the VS Code extension too. Feedback on TypeScript 7.0 will go a long way, and you can file issues on our issue tracker.

Still, TypeScript 6.0 is a stable release that you should be able to adopt today, and it includes a number of improvements and new features that you can start using right away. We hope this release will be a smooth transition for everyone, and we look forward to hearing about your experiences with it.

Happy Hacking!

– Daniel Rosenwasser and the TypeScript Team

Daniel Rosenwasser is the product manager of the TypeScript team. He has a passion for programming languages, compilers, and great developer tooling.