Modules - Theory

Scripts and modules in JavaScript

In the early days of JavaScript, when the language only ran in browsers, there were no modules, but it was still possible to split the JavaScript for a web page into multiple files by using multiple script tags in HTML:

html
<html>
  <head>
    <script src="a.js"></script>
    <script src="b.js"></script>
  </head>
  <body></body>
</html>

This approach had some downsides, especially as web pages grew larger and more complex. In particular, all scripts loaded onto the same page share the same scope—appropriately called the “global scope”—meaning the scripts had to be very careful not to overwrite each others’ variables and functions.

Any system that solves this problem by giving files their own scope while still providing a way to make bits of code available to other files can be called a “module system.” (It may sound obvious to say that each file in a module system is called a “module,” but the term is often used to contrast with script files, which run outside a module system, in a global scope.)

There are many module systems, and TypeScript supports emitting several, but this documentation will focus on the two most important systems today: ECMAScript modules (ESM) and CommonJS (CJS).

ECMAScript Modules (ESM) is the module system built into the language, supported in modern browsers and in Node.js since v12. It uses dedicated import and export syntax:

js
// a.js
export default "Hello from a.js";
js
// b.js
import a from "./a.js";
console.log(a); // 'Hello from a.js'

CommonJS (CJS) is the module system that originally shipped in Node.js, before ESM was part of the language specification. It’s still supported in Node.js alongside ESM. It uses plain JavaScript objects and functions named exports and require:

js
// a.js
exports.message = "Hello from a.js";
js
// b.js
const a = require("./a");
console.log(a.message); // 'Hello from a.js'

Accordingly, when TypeScript detects that a file is a CommonJS or ECMAScript module, it starts by assuming that file will have its own scope. Beyond that, though, the compiler’s job gets a little more complicated.

TypeScript’s job concerning modules

The TypeScript compiler’s chief goal is to prevent certain kinds of runtime errors by catching them at compile time. With or without modules involved, the compiler needs to know about the code’s intended runtime environment—what globals are available, for example. When modules are involved, there are several additional questions the compiler needs to answer in order to do its job. Let’s use a few lines of input code as an example to think about all the information needed to analyze it:

ts
import sayHello from "greetings";
sayHello("world");

To check this file, the compiler needs to know the type of sayHello (is it a function that can accept one string argument?), which opens quite a few additional questions:

  1. Will the module system load this TypeScript file directly, or will it load a JavaScript file that I (or another compiler) generate from this TypeScript file?
  2. What kind of module does the module system expect to find, given the file name it will load and its location on disk?
  3. If output JavaScript is being emitted, how will the module syntax present in this file be transformed in the output code?
  4. Where will the module system look to find the module specified by "greetings"? Will the lookup succeed?
  5. What kind of module is the file resolved by that lookup?
  6. Does the module system allow the kind of module detected in (2) to reference the kind of module detected in (5) with the syntax decided in (3)?
  7. Once the "greetings" module has been analyzed, what piece of that module is bound to sayHello?

Notice that all of these questions depend on characteristics of the host—the system that ultimately consumes the output JavaScript (or raw TypeScript, as the case may be) to direct its module loading behavior, typically either a runtime (like Node.js) or bundler (like Webpack).

The ECMAScript specification defines how ESM imports and exports link up with each other, but it doesn’t specify how the file lookup in (4), known as module resolution, happens, and it doesn’t say anything about other module systems like CommonJS. So runtimes and bundlers, especially those that want to support both ESM and CJS, have a lot of freedom to design their own rules. Consequently, the way TypeScript should answer the questions above can vary dramatically depending on where the code is intended to run. There’s no single right answer, so the compiler must be told the rules through configuration options.

The other key idea to keep in mind is that TypeScript almost always thinks about these questions in terms of its output JavaScript files, not its input TypeScript (or JavaScript!) files. Today, some runtimes and bundlers support loading TypeScript files directly, and in those cases, it doesn’t make sense to think about separate input and output files. Most of this document discusses cases where TypeScript files are compiled to JavaScript files, which in turn are loaded by the runtime module system. Examining these cases is essential for building an understanding of the compiler’s options and behavior—it’s easier to start there and simplify when thinking about esbuild, Bun, and other TypeScript-first runtimes and bundlers. So for now, we can summarize TypeScript’s job when it comes to modules in terms of output files:

Understand the rules of the host enough

  1. to compile files into a valid output module format,
  2. to ensure that imports in those outputs will resolve successfully, and
  3. to know what type to assign to imported names.

Who is the host?

Before we move on, it’s worth making sure we’re on the same page about the term host, because it will come up frequently. We defined it before as “the system that ultimately consumes the output code to direct its module loading behavior.” In other words, it’s the system outside of TypeScript that TypeScript’s module analysis tries to model:

  • When the output code (whether produced by tsc or a third-party transpiler) is run directly in a runtime like Node.js, the runtime is the host.
  • When there is no “output code” because a runtime consumes TypeScript files directly, the runtime is still the host.
  • When a bundler consumes TypeScript inputs or outputs and produces a bundle, the bundler is the host, because it looked at the original set of imports/requires, looked up what files they referenced, and produced a new file or set of files where the original imports and requires are erased or transformed beyond recognition. (That bundle itself might comprise modules, and the runtime that runs it will be its host, but TypeScript doesn’t know about anything that happens post-bundler.)
  • If another transpiler, optimizer, or formatter runs on TypeScript’s outputs, it’s not a host that TypeScript cares about, as long as it leaves the imports and exports it sees alone.
  • When loading modules in a web browser, the behaviors TypeScript needs to model are actually split between the web server and the module system running in the browser. The browser’s JavaScript engine (or a script-based module-loading framework like RequireJS) controls what module formats are accepted, while the web server decides what file to send when one module triggers a request to load another.
  • The TypeScript compiler itself is not a host, because it does not provide any behavior related to modules beyond trying to model other hosts.

The module output format

In any project, the first question about modules we need to answer is what kinds of modules the host expects, so TypeScript can set its output format for each file to match. Sometimes, the host only supports one kind of module—ESM in the browser, or CJS in Node.js v11 and earlier, for example. Node.js v12 and later accepts both CJS and ES modules, but uses file extensions and package.json files to determine what format each file should be, and throws an error if the file’s contents don’t match the expected format.

The module compiler option provides this information to the compiler. Its primary purpose is to control the module format of any JavaScript that gets emitted during compilation, but it also serves to inform the compiler about how the module kind of each file should be detected, how different module kinds are allowed to import each other, and whether features like import.meta and top-level await are available. So, even if a TypeScript project is using noEmit, choosing the right setting for module still matters. As we established earlier, the compiler needs an accurate understanding of the module system so it can type check (and provide IntelliSense for) imports. See Choosing compiler options for guidance on choosing the right module setting for your project.

The available module settings are

  • node16: Reflects the module system of Node.js v16+, which supports ES modules and CJS modules side-by-side with particular interoperability and detection rules.
  • nodenext: Currently identical to node16, but will be a moving target reflecting the latest Node.js versions as Node.js’s module system evolves.
  • es2015: Reflects the ES2015 language specification for JavaScript modules (the version that first introduced import and export to the language).
  • es2020: Adds support for import.meta and export * as ns from "mod" to es2015.
  • es2022: Adds support for top-level await to es2020.
  • esnext: Currently identical to es2022, but will be a moving target reflecting the latest ECMAScript specifications, as well as module-related Stage 3+ proposals that are expected to be included in upcoming specification versions.
  • commonjs, system, amd, and umd: Each emits everything in the module system named, and assumes everything can be successfully imported into that module system. These are no longer recommended for new projects and will not be covered in detail by this documentation.

Node.js’s rules for module format detection and interoperability make it incorrect to specify module as esnext or commonjs for projects that run in Node.js, even if all files emitted by tsc are ESM or CJS, respectively. The only correct module settings for projects that intend to run in Node.js are node16 and nodenext. While the emitted JavaScript for an all-ESM Node.js project might look identical between compilations using esnext and nodenext, the type checking can differ. See the reference section on nodenext for more details.

Module format detection

Node.js understands both ES modules and CJS modules, but the format of each file is determined by its file extension and the type field of the first package.json file found in a search of the file’s directory and all ancestor directories:

  • .mjs and .cjs files are always interpreted as ES modules and CJS modules, respectively.
  • .js files are interpreted as ES modules if the nearest package.json file contains a type field with the value "module". If there is no package.json file, or if the type field is missing or has any other value, .js files are interpreted as CJS modules.

If a file is determined to be an ES module by these rules, Node.js will not inject the CommonJS module and require objects into the file’s scope during evaluation, so a file that tries to use them will cause a crash. Conversely, if a file is determined to be a CJS module, import and export declarations in the file will cause a syntax error crash.

When the module compiler option is set to node16 or nodenext, TypeScript applies this same algorithm to the project’s input files to determine the module kind of each corresponding output file. Let’s look at how module formats are detected in an example project that uses --module nodenext:

Input file name Contents Output file name Module kind Reason
/package.json {}
/main.mts /main.mjs ESM File extension
/utils.cts /utils.cjs CJS File extension
/example.ts /example.js CJS No "type": "module" in package.json
/node_modules/pkg/package.json { "type": "module" }
/node_modules/pkg/index.d.ts ESM "type": "module" in package.json
/node_modules/pkg/index.d.cts CJS File extension

When the input file extension is .mts or .cts, TypeScript knows to treat that file as an ES module or CJS module, respectively, because Node.js will treat the output .mjs file as an ES module or the output .cjs file as a CJS module. When the input file extension is .ts, TypeScript has to consult the nearest package.json file to determine the module format, because this is what Node.js will do when it encounters the output .js file. (Notice that the same rules apply to the .d.cts and .d.ts declaration files in the pkg dependency: though they will not produce an output file as part of this compilation, the presence of a .d.ts file implies the existence of a corresponding .js file—perhaps created when the author of the pkg library ran tsc on an input .ts file of their own—which Node.js must interpret as an ES module, due to its .js extension and the presence of the "type": "module" field in /node_modules/pkg/package.json. Declaration files are covered in more detail in a later section.)

The detected module format of input files is used by TypeScript to ensure it emits the output syntax that Node.js expects in each output file. If TypeScript were to emit /example.js with import and export statements in it, Node.js would crash when parsing the file. If TypeScript were to emit /main.mjs with require calls, Node.js would crash during evaluation. Beyond emit, the module format is also used to determine rules for type checking and module resolution, which we’ll discuss in the following sections.

It’s worth mentioning again that TypeScript’s behavior in --module node16 and --module nodenext is entirely motivated by Node.js’s behavior. Since TypeScript’s goal is to catch potential runtime errors at compile time, it needs a very accurate model of what will happen at runtime. This fairly complex set of rules for module kind detection is necessary for checking code that will run in Node.js, but may be overly strict or just incorrect if applied to non-Node.js hosts.

Input module syntax

It’s important to note that the input module syntax seen in input source files is somewhat decoupled from the output module syntax emitted to JS files. That is, a file with an ESM import:

ts
import { sayHello } from "greetings";
sayHello("world");

might be emitted in ESM format exactly as-is, or might be emitted as CommonJS:

ts
Object.defineProperty(exports, "__esModule", { value: true });
const greetings_1 = require("greetings");
(0, greetings_1.sayHello)("world");

depending on the module compiler option (and any applicable module format detection rules, if the module option supports more than one kind of module). In general, this means that looking at the contents of an input file isn’t enough to determine whether it’s an ES module or a CJS module.

Today, most TypeScript files are authored using ESM syntax (import and export statements) regardless of the output format. This is largely a legacy of the long road ESM has taken to widespread support. ECMAScript modules were standardized in 2015, were supported in most browsers by 2017, and landed in Node.js v12 in 2019. During much of this window, it was clear that ESM was the future of JavaScript modules, but very few runtimes could consume it. Tools like Babel made it possible for JavaScript to be authored in ESM and downleveled to another module format that could be used in Node.js or browsers. TypeScript followed suit, adding support for ES module syntax and softly discouraging the use of the original CommonJS-inspired import fs = require("fs") syntax in the 1.5 release.

The upside of this “author ESM, output anything” strategy was that TypeScript could use standard JavaScript syntax, making the authoring experience familiar to newcomers, and (theoretically) making it easy for projects to start targeting ESM outputs in the future. There are three significant downsides, which became fully apparent only after ESM and CJS modules were allowed to coexist and interoperate in Node.js:

  1. Early assumptions about how ESM/CJS interoperability would work in Node.js turned out to be wrong, and today, interoperability rules differ between Node.js and bundlers. Consequently, the configuration space for modules in TypeScript is large.
  2. When the syntax in input files all looks like ESM, it’s easy for an author or code reviewer to lose track of what kind of module a file is at runtime. And because of Node.js’s interoperability rules, what kind of module each file is became very important.
  3. When input files are written in ESM, the syntax in type declaration outputs (.d.ts files) looks like ESM too. But because the corresponding JavaScript files could have been emitted in any module format, TypeScript can’t tell what kind of module a file is just by looking at the contents of its type declarations. And again, because of the nature of ESM/CJS interoperability, TypeScript has to know what kind of module everything is in order to provide correct types and prevent imports that will crash.

In TypeScript 5.0, a new compiler option called verbatimModuleSyntax was introduced to help TypeScript authors know exactly how their import and export statements will be emitted. When enabled, the flag requires imports and exports in input files to be written in the form that will undergo the least amount of transformation before emit. So if a file will be emitted as ESM, imports and exports must be written in ESM syntax; if a file will be emitted as CJS, it must be written in the CommonJS-inspired TypeScript syntax (import fs = require("fs") and export = {}). This setting is particularly recommended for Node.js projects that use mostly ESM, but have a select few CJS files. It is not recommended for projects that currently target CJS, but may want to target ESM in the future.

ESM and CJS interoperability

Can an ES module import a CommonJS module? If so, does a default import link to exports or exports.default? Can a CommonJS module require an ES module? CommonJS isn’t part of the ECMAScript specification, so runtimes, bundlers, and transpilers have been free to make up their own answers to these questions since ESM was standardized in 2015, and as such no standard set of interoperability rules exist. Today, most runtimes and bundlers broadly fall into one of three categories:

  1. ESM-only. Some runtimes, like browser engines, only support what’s actually a part of the language: ECMAScript Modules.
  2. Bundler-like. Before any major JavaScript engine could run ES modules, Babel allowed developers to write them by transpiling them to CommonJS. The way these ESM-transpiled-to-CJS files interacted with hand-written-CJS files implied a set of permissive interoperability rules that have become the de facto standard for bundlers and transpilers.
  3. Node.js. In Node.js, CommonJS modules cannot load ES modules synchronously (with require); they can only load them asynchronously with dynamic import() calls. ES modules can default-import CJS modules, which always binds to exports. (This means that a default import of a Babel-like CJS output with __esModule behaves differently between Node.js and some bundlers.)

TypeScript needs to know which of these rule sets to assume in order to provide correct types on (particularly default) imports and to error on imports that will crash at runtime. When the module compiler option is set to node16 or nodenext, Node.js’s rules are enforced. All other module settings, combined with the esModuleInterop option, result in bundler-like interop in TypeScript. (While using --module esnext does prevent you from writing CommonJS modules, it does not prevent you from importing them as dependencies. There’s currently no TypeScript setting that can guard against an ES module importing a CommonJS module, as would be appropriate for direct-to-browser code.)

Module specifiers are not transformed

While the module compiler option can transform imports and exports in input files to different module formats in output files, the module specifier (the string from which you import, or pass to require) is always emitted as-written. For example, an input like:

ts
import { add } from "./math.mjs";
add(1, 2);

might be emitted as either:

ts
import { add } from "./math.mjs";
add(1, 2);

or:

ts
const math_1 = require("./math.mjs");
math_1.add(1, 2);

depending on the module compiler option, but the module specifier will always be "./math.mjs". There is no compiler option that enables transforming, substituting, or rewriting module specifiers. Consequently, module specifiers must be written in a way that works for the code’s target runtime or bundler, and it’s TypeScript’s job to understand those output-relative specifiers. The process of finding the file referenced by a module specifier is called module resolution.

Module resolution

Let’s return to our first example and review what we’ve learned about it so far:

ts
import sayHello from "greetings";
sayHello("world");

So far, we’ve discussed how the host’s module system and TypeScript’s module compiler option might impact this code. We know that the input syntax looks like ESM, but the output format depends on the module compiler option, potentially the file extension, and package.json "type" field. We also know that what sayHello gets bound to, and even whether the import is even allowed, may vary depending on the module kinds of this file and the target file. But we haven’t yet discussed how to find the target file.

Module resolution is host-defined

While the ECMAScript specification defines how to parse and interpret import and export statements, it leaves module resolution up to the host. If you’re creating a hot new JavaScript runtime, you’re free to create a module resolution scheme like:

ts
import monkey from "🐒"; // Looks for './eats/bananas.js'
import cow from "🐄";    // Looks for './eats/grass.js'
import lion from "🦁";   // Looks for './eats/you.js'

and still claim to implement “standards-compliant ESM.” Needless to say, TypeScript would have no idea what types to assign to monkey, cow, and lion without built-in knowledge of this runtime’s module resolution algorithm. Just as module informs the compiler about the host’s expected module format, moduleResolution, along with a few customization options, specify the algorithm the host uses to resolve module specifiers to files. This also clarifies why TypeScript doesn’t modify import specifiers during emit: the relationship between an import specifier and a file on disk (if one even exists) is host-defined, and TypeScript is not a host.

The available moduleResolution options are:

  • classic: TypeScript’s oldest module resolution mode, this is unfortunately the default when module is set to anything other than commonjs, node16, or nodenext. It was probably made to provide best-effort resolution for a wide range of RequireJS configurations. It should not be used for new projects (or even old projects that don’t use RequireJS or another AMD module loader), and is scheduled for deprecation in TypeScript 6.0.
  • node10: Formerly known as node, this is the unfortunate default when module is set to commonjs. It’s a pretty good model of Node.js versions older than v12, and sometimes it’s a passable approximation of how most bundlers do module resolution. It supports looking up packages from node_modules, loading directory index.js files, and omitting .js extensions in relative module specifiers. Because Node.js v12 introduced different module resolution rules for ES modules, though, it’s a very bad model of modern versions of Node.js. It should not be used for new projects.
  • node16: This is the counterpart of --module node16 and is set by default with that module setting. Node.js v12 and later support both ESM and CJS, each of which uses its own module resolution algorithm. In Node.js, module specifiers in import statements and dynamic import() calls are not allowed to omit file extensions or /index.js suffixes, while module specifiers in require calls are. This module resolution mode understands and enforces this restriction where necessary, as determined by the module format detection rules instated by --module node16. (For node16 and nodenext, module and moduleResolution go hand-in-hand: setting one to node16 or nodenext while setting the other to something else has unsupported behavior and may be an error in the future.)
  • nodenext: Currently identical to node16, this is the counterpart of --module nodenext and is set by default with that module setting. It’s intended to be a forward-looking mode that will support new Node.js module resolution features as they’re added.
  • bundler: Node.js v12 introduced some new module resolution features for importing npm packages—the "exports" and "imports" fields of package.json—and many bundlers adopted those features without also adopting the stricter rules for ESM imports. This module resolution mode provides a base algorithm for code targeting a bundler. It supports package.json "exports" and "imports" by default, but can be configured to ignore them. It requires setting module to esnext.

TypeScript imitates the host’s module resolution, but with types

Remember the three components of TypeScript’s job concerning modules?

  1. Compile files into a valid output module format
  2. Ensure that imports in those outputs will resolve successfully
  3. Know what type to assign to imported names.

Module resolution is needed to accomplish last two. But when we spend most of our time working in input files, it can be easy to forget about (2)—that a key component of module resolution is validating that the imports or require calls in the output files, containing the same module specifiers as the input files, will actually work at runtime. Let’s look at a new example with multiple files:

ts
// @Filename: math.ts
export function add(a: number, b: number) {
  return a + b;
}
// @Filename: main.ts
import { add } from "./math";
add(1, 2);

When we see the import from "./math", it might be tempting to think, “This is how one TypeScript file refers to another. The compiler follows this (extensionless) path in order to assign a type to add.”

A simple flowchart diagram. A file (rectangle node) main.ts resolves (labeled arrow) through module specifier './math' to another file math.ts.

This isn’t entirely wrong, but the reality is deeper. The resolution of "./math" (and subsequently, the type of add) need to reflect the reality of what happens at runtime to the output files. A more robust way to think about this process would look like this:

A flowchart diagram with two groups of files: Input files and Output files. main.ts (an input file) maps to output file main.js, which resolves through the module specifier "./math" to math.js (another output file), which maps back to the input file math.ts.

This model makes it clear that for TypeScript, module resolution is mostly a matter of accurately modeling the host’s module resolution algorithm between output files, with a little bit of remapping applied to find type information. Let’s look at another example that appears unintuitive through the lens of the simple model, but makes perfect sense with the robust model:

ts
// @moduleResolution: node16
// @rootDir: src
// @outDir: dist
// @Filename: src/math.mts
export function add(a: number, b: number) {
  return a + b;
}
// @Filename: src/main.mts
import { add } from "./math.mjs";
add(1, 2);

Node.js ESM import declarations use a strict module resolution algorithm that requires relative paths to include file extensions. When we only think about input files, it’s a little strange that "./math.mjs" seems to resolve to math.mts. Since we’re using an outDir to put compiled outputs in a different directory, math.mjs doesn’t even exist next to main.mts! Why should this resolve? With our new mental model, it’s no problem:

A flowchart diagram with identical structure to the one above. There are two groups of files: Input files and Output files. src/main.mts (an input file) maps to output file dist/main.mjs, which resolves through module specifier "./math.mjs" to dist/math.mjs (another output file), which maps back to input file src/math.mts.

Understanding this mental model may not immediately eliminate the strangeness of seeing output file extensions in input files, and it’s natural to think in terms of shortcuts: "./math.mjs" refers to the input file math.mts. I have to write the output extension, but the compiler knows to look for .mts when I write .mjs. This shortcut is even how the compiler works internally, but the more robust mental model explains why module resolution in TypeScript works this way: given the constraint that the module specifier in the output file will be the same as the module specifier in the input file, this is the only process that accomplishes our two goals of validating output files and assigning types.

The role of declaration files

In the previous example, we saw the “remapping” part of module resolution working between input and output files. But what happens when we import library code? Even if the library was written in TypeScript, it may not have published its source code. If we can’t rely on mapping the library’s JavaScript files back to a TypeScript file, we can verify that our import works at runtime, but how do we accomplish our second goal of assigning types?

This is where declaration files (.d.ts, .d.mts, etc.) come into play. The best way to understand how declaration files are interpreted is to understand where they come from. When you run tsc --declaration on an input file, you get one output JavaScript file and one output declaration file:

A diagram showing the relationship between different file types. A .ts file (top) has two arrows labeled 'generates' flowing to a .js file (bottom left) and a .d.ts file (bottom right). Another arrow labeled 'implies' points from the .d.ts file to the .js file.

Because of this relationship, the compiler assumes that wherever it sees a declaration file, there is a corresponding JavaScript file that is perfectly described by the type information in the declaration file. For performance reasons, in every module resolution mode, the compiler always looks for TypeScript and declaration files first, and if it finds one, it doesn’t continue looking for the corresponding JavaScript file. If it finds a TypeScript input file, it knows a JavaScript file will exist after compilation, and if it finds a declaration file, it knows a compilation (perhaps someone else’s) already happened and created a JavaScript file at the same time as the declaration file.

The declaration file tells the compiler not only that a JavaScript file exists, but also what its name and extension are:

Declaration file extension JavaScript file extension TypeScript file extension
.d.ts .js .ts
.d.ts .js .tsx
.d.mts .mjs .mts
.d.cts .cjs .cts
.d.*.ts .*

The last row expresses that non-JS files can be typed with the allowArbitraryExtensions compiler option to support cases where the module system supports importing non-JS files as JavaScript objects. For example, a file named styles.css can be represented by a declaration file named styles.d.css.ts.

“But wait! Plenty of declaration files are written by hand, not generated by tsc. Ever heard of DefinitelyTyped?” you might object. And it’s true—hand-writing declaration files, or even moving/copying/renaming them to represent outputs of an external build tool, is a dangerous, error-prone venture. DefinitelyTyped contributors and authors of typed libraries not using tsc to generate both JavaScript and declaration files should ensure that every JavaScript file has a sibling declaration file with the same name and matching extension. Breaking from this structure can lead to false-positive TypeScript errors for end users. The npm package @arethetypeswrong/cli can help catch and explain these errors before they’re published.

Module resolution for bundlers, TypeScript runtimes, and Node.js loaders

So far, we’ve really emphasized the distinction between input files and output files. Recall that when specifying a file extension on a relative module specifier, TypeScript typically makes you use the output file extension:

ts
// @Filename: src/math.ts
export function add(a: number, b: number) {
  return a + b;
}
// @Filename: src/main.ts
import { add } from "./math.ts";
//                  ^^^^^^^^^^^
// An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.

This restriction applies since TypeScript won’t rewrite the extension to .js, and if "./math.ts" appears in an output JS file, that import won’t resolve to another JS file at runtime. TypeScript really wants to prevent you from generating an unsafe output JS file. But what if there is no output JS file? What if you’re in one of these situations:

  • You’re bundling this code, the bundler is configured to transpile TypeScript files in-memory, and it will eventually consume and erase all the imports you’ve written to produce a bundle.
  • You’re running this code directly in a TypeScript runtime like Deno or Bun.
  • You’re using ts-node, tsx, or another transpiling loader for Node.

In these cases, you can turn on noEmit (or emitDeclarationOnly) and allowImportingTsExtensions to disable emitting unsafe JavaScript files and silence the error on .ts-extensioned imports.

With or without allowImportingTsExtensions, it’s still important to pick the most appropriate moduleResolution setting for the module resolution host. For bundlers and the Bun runtime, it’s bundler. These module resolvers were inspired by Node.js, but didn’t adopt the strict ESM resolution algorithm that disables extension searching that Node.js applies to imports. The bundler module resolution setting reflects this, enabling package.json "exports" support like node16 and nodenext, while always allowing extensionless imports. See Choosing compiler options for more guidance.

Module resolution for libraries

When compiling an app, you choose the moduleResolution option for a TypeScript project based on who the module resolution host is. When compiling a library, you don’t know where the output code will run, but you’d like it to run in as many places as possible. Using "module": "nodenext" (along with the implied "moduleResolution": "nodenext") is the best bet for maximizing the compatibility of the output JavaScript’s module specifiers, since it will force you to comply with Node.js’s stricter rules for import module resolution. Let’s look at what would happen if a library were to compile with "moduleResolution": "bundler" (or worse, "node10"):

ts
export * from "./utils";

Assuming ./utils.ts (or ./utils/index.ts) exists, a bundler would be fine with this code, so "moduleResolution": "bundler" doesn’t complain. Compiled with "module": "esnext", the output JavaScript for this export statement will look exactly the same as the input. If that JavaScript were published to npm, it would be usable by projects that use a bundler, but it would cause an error when run in Node.js:

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '.../node_modules/dependency/utils' imported from .../node_modules/dependency/index.js
Did you mean to import ./utils.js?

On the other hand, if we had written:

ts
export * from "./utils.js";

This would produce output that works both in Node.js and in bundlers.

In short, "moduleResolution": "bundler" is infectious, allowing code that only works in bundlers to be produced. Likewise, "moduleResolution": "nodenext" is only checking that the output works in Node.js, but in most cases, module code that works in Node.js will work in other runtimes and in bundlers.

Of course, this guidance can only apply in cases where the library ships outputs from tsc. If the library is being bundled before shipping, "moduleResolution": "bundler" may be acceptable. Any build tool that changes the module format or module specifiers to produce the final build of the library bears the responsibility of ensuring the safety and compatibility of the product’s module code, and tsc can no longer contribute to that task, since it can’t know what module code will exist at runtime.

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JBJake Bailey (6)

Last updated: Aug 26, 2024