docs.rodeo

MDN Web Docs mirror

import()

{{jsSidebar("Operators")}} 

The import() syntax, commonly called dynamic import, is a function-like expression that allows loading an ECMAScript module asynchronously and dynamically into a potentially non-module environment.

Unlike the declaration-style counterpart, dynamic imports are only evaluated when needed, and permit greater syntactic flexibility.

Syntax

import(moduleName)
import(moduleName, options)

The import() call is a syntax that closely resembles a function call, but import itself is a keyword, not a function. You cannot alias it like const myImport = import, which will throw a {{jsxref("SyntaxError")}} .

Trailing commas are only allowed if the runtime also supports options. Check browser compatibility.

Parameters

Return value

Returns a promise which:

Note: import() never synchronously throws an error.

Description

The import declaration syntax (import something from "somewhere") is static and will always result in the imported module being evaluated at load time. Dynamic imports allow one to circumvent the syntactic rigidity of import declarations and load a module conditionally or on demand. The following are some reasons why you might need to use dynamic import:

Use dynamic import only when necessary. The static form is preferable for loading initial dependencies, and can benefit more readily from static analysis tools and tree shaking.

If your file is not run as a module (if it’s referenced in an HTML file, the script tag must have type="module"), you will not be able to use static import declarations. On the other hand, the asynchronous dynamic import syntax is always available, allowing you to import modules into non-module environments.

The options parameter allows different kinds of import options. For example, import attributes:

import("./data.json", { with: { type: "json" } });

Dynamic module import is not permitted in all execution contexts. For example, import() can be used in the main thread, a shared worker, or a dedicated worker, but will throw if called within a service worker or a worklet.

Module namespace object

A module namespace object is an object that describes all exports from a module. It is a static object that is created when the module is evaluated. There are two ways to access the module namespace object of a module: through a namespace import (import * as name from moduleName), or through the fulfillment value of a dynamic import.

The module namespace object is a sealed object with null prototype. This means all string keys of the object correspond to the exports of the module and there are never extra keys. All keys are enumerable in lexicographic order (i.e. the default behavior of Array.prototype.sort()), with the default export available as a key called default. In addition, the module namespace object has a [Symbol.toStringTag] property with the value "Module", used in {{jsxref("Object.prototype.toString()")}} .

The string properties are non-configurable and writable when you use {{jsxref("Object.getOwnPropertyDescriptors()")}}  to get their descriptors. However, they are effectively read-only, because you cannot re-assign a property to a new value. This behavior mirrors the fact that static imports create “live bindings” — the values can be re-assigned by the module exporting them, but not by the module importing them. The writability of the properties reflects the possibility of the values changing, because non-configurable and non-writable properties must be constant. For example, you can re-assign the exported value of a variable, and the new value can be observed in the module namespace object.

Each (normalized) module specifier corresponds to a unique module namespace object, so the following is generally true:

import * as mod from "/my-module.js";

import("/my-module.js").then((mod2) => {
  console.log(mod === mod2); // true
});

Except in one curious case: because a promise never fulfills to a thenable, if the my-module.js module exports a function called then(), that function will automatically get called when the dynamic import’s promise is fulfilled, as part of the promise resolution process.

// my-module.js
export function then(resolve) {
  console.log("then() called");
  resolve(1);
}

// main.js
import * as mod from "/my-module.js";

import("/my-module.js").then((mod2) => {
  // Logs "then() called"
  console.log(mod === mod2); // false
});

[!WARNING] Do not export a function called then() from a module. This will cause the module to behave differently when imported dynamically than when imported statically.

This aggressive caching ensures that a piece of JavaScript code is never executed more than once, even if it is imported multiple times. Future imports don’t even result in HTTP requests or disk access. If you do need to re-import and re-evaluate a module without restarting the entire JavaScript environment, one possible trick is to use a unique query parameter in the module specifier. This works in non-browser runtimes that support URL specifiers too.

import("/my-module.js?t=" + Date.now());

Note that this can lead to memory leaks in a long-running application, because the engine cannot safely garbage-collect any module namespace objects. Currently, there is no way to manually clear the cache of module namespace objects.

Module namespace object caching only applies to modules that are loaded and linked successfully. A module is imported in three steps: loading (fetching the module), linking (mostly, parsing the module), and evaluating (executing the parsed code). Only evaluation failures are cached; if a module fails to load or link, the next import may try to load and link the module again. The browser may or may not cache the result of the fetch operation, but it should follow typical HTTP semantics, so handling such network failures should not be different from handling {{domxref("Window/fetch", "fetch()")}}  failures.

Examples

Import a module for its side effects only

(async () => {
  if (somethingIsTrue) {
    // import module for side effects
    await import("/modules/my-module.js");
  }
})();

If your project uses packages that export ESM, you can also import them for side effects only. This will run the code in the package entry point file (and any files it imports) only.

Importing defaults

If you are destructuring the imported module namespace object, then you must rename the default key because default is a reserved word.

(async () => {
  if (somethingIsTrue) {
    const {
      default: myDefault,
      foo,
      bar,
    } = await import("/modules/my-module.js");
  }
})();

Importing on-demand in response to user action

This example shows how to load functionality on to a page based on a user action, in this case a button click, and then call a function within that module. This is not the only way to implement this functionality. The import() function also supports await.

const main = document.querySelector("main");
for (const link of document.querySelectorAll("nav > a")) {
  link.addEventListener("click", (e) => {
    e.preventDefault();

    import("/modules/my-module.js")
      .then((module) => {
        module.loadPageInto(main);
      })
      .catch((err) => {
        main.textContent = err.message;
      });
  });
}

Importing different modules based on environment

In processes such as server-side rendering, you may need to load different logic on server or in browser because they interact with different globals or modules (for example, browser code has access to web APIs like document and navigator, while server code has access to the server file system). You can do so through a conditional dynamic import.

let myModule;

if (typeof window === "undefined") {
  myModule = await import("module-used-on-server");
} else {
  myModule = await import("module-used-in-browser");
}

Importing modules with a non-literal specifier

Dynamic imports allow any expression as the module specifier, not necessarily string literals.

Here, we load 10 modules, /modules/module-0.js, /modules/module-1.js, etc., concurrently, and call the load functions that each one exports.

Promise.all(
  Array.from({ length: 10 }).map(
    (_, index) => import(`/modules/module-${index}.js`),
  ),
).then((modules) => modules.forEach((module) => module.load()));

Specifications

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

In this article

View on MDN