How can I conditionally import an ES6 module?

Question

I need to do something like:

if (condition) {
    import something from 'something';
}
// ...
if (something) {
    something.doStuff();
}

The above code does not compile; it throws SyntaxError: ... 'import' and 'export' may only appear at the top level.

I tried using System.import as shown here, but I don't know where System comes from. Is it an ES6 proposal that didn't end up being accepted? The link to "programmatic API" from that article dumps me to a deprecated docs page.

Answer 1

We do have dynamic imports proposal now with ECMA. This is in stage 3. This is also available as babel-preset.

Following is way to do conditional rendering as per your case.

if (condition) {
    import('something')
    .then((something) => {
       console.log(something.something);
    });
}

This basically returns a promise. Resolution of promise is expected to have the module. The proposal also have other features like multiple dynamic imports, default imports, js file import etc. You can find more information about dynamic imports here.

Answer 2

If you'd like, you could use require. This is a way to have a conditional require statement.

let something = null;
let other = null;

if (condition) {
    something = require('something');
    other = require('something').other;
}
if (something && other) {
    something.doStuff();
    other.doOtherStuff();
}

Answer 3

You can't import conditionally, but you can do the opposite: export something conditionally. It depends on your use case, so this work around might not be for you.

You can do:

api.js

import mockAPI from './mockAPI'
import realAPI from './realAPI'

const exportedAPI = shouldUseMock ? mockAPI : realAPI
export default exportedAPI

apiConsumer.js

import API from './api'
...

I use that to mock analytics libs like mixpanel, etc... because I can't have multiple builds or our frontend currently. Not the most elegant, but works. I just have a few 'if' here and there depending on the environment because in the case of mixpanel, it needs initialization.

Answer 4

2020 Update

You can now call the import keyword as a function (i.e. import()) to load a module at runtime. It returns a Promise that resolves to an object with the module exports.

Example:

const mymodule = await import('modulename');
const foo = mymodule.default; // Default export
const bar = mymodule.bar; // Named export

or:

import('modulename')
    .then(mymodule => {
        const foo = mymodule.default; // Default export
        const bar = mymodule.bar; // Named export
    });

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports

Answer 5

Looks like the answer is that, as of now, you can't.

http://exploringjs.com/es6/ch_modules.html#sec_module-loader-api

I think the intent is to enable static analysis as much as possible, and conditionally imported modules break that. Also worth mentioning -- I'm using Babel, and I'm guessing that System is not supported by Babel because the module loader API didn't become an ES6 standard.

Answer 6

Important difference if you use dynamic import Webpack mode eager:

if (normalCondition) {
  // this will be included to bundle, whether you use it or not
  import(...);
}

if (process.env.SOMETHING === 'true') {
  // this will not be included to bundle, if SOMETHING is not 'true'
  import(...);
}

Answer 7

Import and Export Conditionally in JS

const value = (
    await import(`${condtion ? `./file1.js` : `./file2.js`}`)
).default

export default value

Answer 8

require() is a way to import some module on the run time and it equally qualifies for static analysis like import if used with string literal paths. This is required by bundler to pick dependencies for the bundle.

const defaultOne = require('path/to/component').default;
const NamedOne = require('path/to/component').theName;

For dynamic module resolution with complete static analysis support, first index modules in an indexer(index.js) and import indexer in host module.

// index.js
export { default as ModuleOne } from 'path/to/module/one';
export { default as ModuleTwo } from 'path/to/module/two';
export { SomeNamedModule } from 'path/to/named/module';

// host.js
import * as indexer from 'index';
const moduleName = 'ModuleOne';
const Module = require(indexer[moduleName]);

Answer 9

Conditional imports could also be achieved with a ternary and require()s:

const logger = DEBUG ? require('dev-logger') : require('logger');

This example was taken from the ES Lint global-require docs: https://eslint.org/docs/rules/global-require

Answer 10

obscuring it in an eval worked for me, hiding it from the static analyzer ...

if (typeof __CLI__ !== 'undefined') {
  eval("require('fs');")
}