import
The static import
statement is
used to import read only live bindings which are exported by
another module.
Imported modules are in strict mode
whether you declare them as such or not. The import
statement cannot be
used in embedded scripts unless such script has a type="module"
. Bindings
imported are called live bindings because they are updated by the module that exported
the binding.
There is also a function-like dynamic import()
, which
does not require scripts of type="module"
.
Backward compatibility can be ensured using attribute nomodule
on the
<script>
tag.
Syntax
import defaultExport from "module-name";
import * as name from "module-name";
import { export1 } from "module-name";
import { export1 as alias1 } from "module-name";
import { export1 , export2 } from "module-name";
import { export1 , export2 as alias2 , [...] } from "module-name";
import defaultExport, { export1 [ , [...] ] } from "module-name";
import defaultExport, * as name from "module-name";
import "module-name";
var promise = import("module-name");
defaultExport
-
Name that will refer to the default export from the module.
module-name
-
The module to import from. This is often a relative or absolute url to the
.js
file containing the module. Certain bundlers may permit or require the use of the extension; 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.
exportN
-
Name of the exports to be imported.
aliasN
-
Names that will refer to the named imports.
Description
The name
parameter is the name of the "module object" which will be used
as a kind of namespace to refer to the exports. The export
parameters
specify individual named exports, while the import * as name
syntax imports
all of them. Below are examples to clarify the syntax.
Import an entire module's contents
This inserts myModule
into the current scope, containing all the exports
from the module in the file located in /modules/my-module.js
.
import * as myModule from '/modules/my-module.js';
Here, accessing the exports means using the module name ("myModule" in this case) as a
namespace. For example, if the module imported above includes an export
doAllTheAmazingThings()
, you would call it like this:
myModule.doAllTheAmazingThings();
Import a single export from a module
Given an object or value named myExport
which has been exported from the
module my-module
either implicitly (because the entire module is exported,
for example using export * from 'another.js'
) or explicitly (using the
export
statement), this inserts myExport
into the current scope.
import {myExport} from '/modules/my-module.js';
Import multiple exports from module
This inserts both foo
and bar
into the current scope.
import {foo, bar} from '/modules/my-module.js';
Import an export with a more convenient alias
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';
Rename multiple exports during import
Import multiple exports from a module with convenient aliases.
import {
reallyReallyLongModuleExportName as shortName,
anotherLongModuleName as short
} from '/modules/my-module.js';
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 works with dynamic imports as well:
(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
It is possible to have a default export
(whether it
is an object, a function, a class, etc.). The import
statement may then be
used to import such defaults.
The simplest version directly imports the default:
import myDefault from '/modules/my-module.js';
It is also possible to use the default syntax with the ones seen above (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 used as a namespace
or
import myDefault, {foo, bar} from '/modules/my-module.js';
// specific, named imports
When importing a default export with dynamic imports, it works a bit differently. You need to destructure and rename the "default" key from the returned object.
(async () => {
if (somethingIsTrue) {
const { default: myDefault, foo, bar } = await import('/modules/my-module.js');
}
})();
Dynamic Imports
The standard import syntax is static and will always result in all code in the imported module being evaluated at load time. In situations where you wish to load a module conditionally or on demand, you can use a dynamic import instead. The following are some reasons why you might need to use dynamic import:
- When importing statically significantly slows the loading of your code and there is a low likelihood that you will need the code you are importing, or you will not need it until a later time.
- When importing statically significantly increases your program's memory usage and there is a low likelihood that you will need the code you are importing.
- When the module you are importing does not exist at load time
- When the import specifier string needs to be constructed dynamically. (Static import only supports static specifiers.)
- When the module being imported has side effects, and you do not want those side effects unless some condition is true. (It is recommended not to have any side effects in a module, but you sometimes cannot control this in your module dependencies.)
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.
To dynamically import a module, the import
keyword may be called as a
function. When used this way, it returns a promise.
import('/modules/my-module.js')
.then((module) => {
// Do something with the module.
});
This form also supports the await
keyword.
let module = await import('/modules/my-module.js');
Examples
Standard Import
The code below shows how to import from a secondary module to assist in processing an AJAX JSON request.
The module: file.js
function getJSON(url, callback) {
let xhr = new XMLHttpRequest();
xhr.onload = function () {
callback(this.responseText)
};
xhr.open('GET', url, true);
xhr.send();
}
export function getUsefulContents(url, callback) {
getJSON(url, data => callback(JSON.parse(data)));
}
The main program: main.js
import { getUsefulContents } from '/modules/file.js';
getUsefulContents('http://www.example.com',
data => { doSomethingUseful(data); });
Dynamic Import
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;
});
});
}
Specifications
Specification |
---|
ECMAScript Language Specification # sec-imports |
Browser compatibility
BCD tables only load in the browser
See also
export
import.meta
- Limin Zhu, Brian Terlson and Microsoft Edge Team: Previewing ES6 Modules and more from ES2015, ES2016 and beyond
- Hacks blog post by Jason Orendorff: ES6 in Depth: Modules
- Hacks blog post by Lin Clark: ES modules: A cartoon deep-dive
- Axel Rauschmayer's book: "Exploring JS: Modules"
- The Modern JavaScript Tutorial(javascript.info): Export and Import