import

import defaultExport from "module-name";
import * as name from "module-name";
import { export1 } from "module-name";
import { export1 as alias1 } from "module-name";
import { default as alias } from "module-name";
import { export1, export2 } from "module-name";
import { export1, export2 as alias2, /* … */ } from "module-name";
import { "string name" as alias } from "module-name";
import defaultExport, { export1, /* … */ } from "module-name";
import defaultExport, * as name from "module-name";
import "module-name";

defaultExport

Name that will refer to the default export from the module. Must be a valid JavaScript identifier.

module-name

The module to import from. The evaluation of the specifier is host-specified. This is often a relative or absolute URL to the .js file containing the module. In Node, extension-less imports often refer to packages in node_modules. Certain bundlers may permit importing files without extensions; check your environment. Only single quoted and double quoted Strings are allowed.

name

Name of the module object that will be used as a kind of namespace when referring to the imports. Must be a valid JavaScript identifier.

exportN

Name of the exports to be imported. The name can be either an identifier or a string literal, depending on what module-name declares to export. If it is a string literal, it must be aliased to a valid identifier.

aliasN

Names that will refer to the named imports. Must be a valid JavaScript identifier.

import declarations can only be present in modules, and only at the top-level (i.e. not inside blocks, functions, etc.). If an import declaration is encountered in non-module contexts (for example, <script> tags without type="module", eval, new Function, which all have "script" or "function body" as parsing goals), a SyntaxError is thrown. To load modules in non-module contexts, use the dynamic import syntax instead.

All imported bindings cannot be in the same scope as any other declaration, including let, const, class, function, var, and import declaration.

import declarations are designed to be syntactically rigid (for example, only string literal specifiers, only permitted at the top-level, all bindings must be identifiers), which allows modules to be statically analyzed and linked before getting evaluated. This is the key to making modules asynchronous by nature, powering features like top-level await.

There are four forms of import declarations:

• Named import: import { export1, export2 } from "module-name";
• Default import: import defaultExport from "module-name";
• Namespace import: import * as name from "module-name";
• Side effect import: import "module-name";

Below are examples to clarify the syntax.

Named import

Given a value named myExport which has been exported from the module my-module either implicitly as export * from "another.js" or explicitly using the export statement, this inserts myExport into the current scope.

import { myExport } from "/modules/my-module.js";

You can import multiple names from the same module.

import { foo, bar } from "/modules/my-module.js";

You can rename an export when importing it. For example, this inserts shortName into the current scope.

import { reallyReallyLongModuleExportName as shortName } from "/modules/my-module.js";

A module may also export a member as a string literal which is not a valid identifier, in which case you must alias it in order to use it in the current module.

// /modules/my-module.js
const a = 1;
export { a as "a-b" };

import { "a-b" as a } from "/modules/my-module.js";

Default import

Default exports need to be imported with the corresponding default import syntax. The simplest version directly imports the default:

import myDefault from "/modules/my-module.js";

Since the default export doesn't explicitly specify a name, you can give the identifier any name you like.

It is also possible to specify a default import with namespace imports or named imports. In such cases, the default import will have to be declared first. For instance:

import myDefault, * as myModule from "/modules/my-module.js";
// myModule.default and myDefault point to the same binding

or

import myDefault, { foo, bar } from "/modules/my-module.js";

Importing a name called default has the same effect as a default import. It is necessary to alias the name because default is a reserved word.

import { default as myDefault } from "/modules/my-module.js";

Namespace import

The following code inserts myModule into the current scope, containing all the exports from the module located at /modules/my-module.js.

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

Here, myModule represents a namespace object which contains all exports as properties. For example, if the module imported above includes an export doAllTheAmazingThings(), you would call it like this:

myModule.doAllTheAmazingThings();

myModule is a sealed object with null prototype. The default export available as a key called default. For more information, see module namespace object.

Import a module for its side effects only

Import an entire module for side effects only, without importing anything. This runs the module's global code, but doesn't actually import any values.

import "/modules/my-module.js";

This is often used for polyfills, which mutate the global variables.

Import declarations are hoisted. In this case, that means that the identifiers the imports introduce are available in the entire module scope, and their side effects are produced before the rest of the module's code runs.

myModule.doAllTheAmazingThings(); // myModule.doAllTheAmazingThings is imported by the next line

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

In this example, we create a re-usable module that exports a function to get all primes within a given range.

// getPrimes.js
/**
 * Returns a list of prime numbers that are smaller than `max`.
 */
export function getPrimes(max) {
  const isPrime = Array.from({ length: max }, () => true);
  isPrime[0] = isPrime[1] = false;
  isPrime[2] = true;
  for (let i = 2; i * i < max; i++) {
    if (isPrime[i]) {
      for (let j = i ** 2; j < max; j += i) {
        isPrime[j] = false;
      }
    }
  }
  return [...isPrime.entries()]
    .filter(([, isPrime]) => isPrime)
    .map(([number]) => number);
}

import { getPrimes } from "/modules/getPrimes.js";

console.log(getPrimes(10)); // [2, 3, 5, 7]

The identifier being imported is a live binding, because the module exporting it may re-assign it and the imported value would change. However, the module importing it cannot re-assign it. Still, any module holding an exported object can mutate the object, and the mutated value can be observed by all other modules importing the same value.

You can also observe the new value through the module namespace object.

// my-module.js
export let myValue = 1;
setTimeout(() => {
  myValue = 2;
}, 500);

// main.js
import { myValue } from "/modules/my-module.js";
import * as myModule from "/modules/my-module.js";

console.log(myValue); // 1
console.log(myModule.myValue); // 1
setTimeout(() => {
  console.log(myValue); // 2; my-module has updated its value
  console.log(myModule.myValue); // 2
  myValue = 3; // TypeError: Assignment to constant variable.
  // The importing module can only read the value but can't re-assign it.
}, 1000);

• export
• import()
• import.meta
• Previewing ES6 Modules and more from ES2015, ES2016 and beyond on blogs.windows.com (2016)
• ES6 in Depth: Modules on hacks.mozilla.org (2015)
• ES modules: A cartoon deep-dive on hacks.mozilla.org (2018)
• Exploring JS, Ch.16: Modules by Dr. Axel Rauschmayer
• Export and Import on javascript.info