import-esm

Import ECMAScript modules in the safe way

Motivation

ECMAScript modules are the official standard format to package JavaScript code for reuse.

Static import is supported since [email protected], you can't use import foo from 'foo' on Node.js@<8.5.0.

Dynamic import is supported since [email protected], which means we will get an SyntaxError like: SyntaxError: Unexpected token import on Node.js@<9.6.0 if you use code like import(url). And you can't avoid this by putting import(url) in a try/catch block.

Install

yarn add import-esm

# or

npm install import-esm

Usage

const importEsm = require('import-esm')

;(async () => {
  try {
    console.log(await importEsm('./foo.mjs'))
    // If your engine support ECMAScript Modules
    //=> [Module] { ... }
  } catch (error) {
    console.error(error)
    // If your engine doesn't support ECMAScript Modules
    //=> Error: ECMAScript Modules are not supported. ...
  }
})()

API

importEsm(url)

Import a Module from url.

Returns a Promise<Module>.

url

Type: string

The module url you want import.

importEsm.check()

Check the ECMAScript Modules support.

Returns a Promise<boolean>.

const importEsm = require('import-esm')

;(async () => {
  console.log(await importEsm.check())
  //=> true
})()

importEsm.checkSync()

The cached result of ECMAScript Modules support check.

Returns a boolean | ''.

Note: Since there is no sync way to detect ECMAScript Modules support, this method returns '' (empty string), if we don't know support result yet. **

To get the real result, you have to compare importEsm.checkSync() with boolean

const importEsm = require('import-esm')

const isEsmSupported = importEsm.checkSync()

if (isEsmSupported === true) {
  // ECMAScript Modules are supported.
} else if (isEsmSupported === false) {
  // ECMAScript Modules are not supported.
} else {
  // We don't know yet.
}

This method is design for easier use in case like this:

const importEsm = require('import-esm')

const importModule = async url => {
  try {
    return await importEsm(url)
  } catch (error) {
    // Runs on every time
    handleNonSupportedError(error)
  }
}

const importModule = async url => {
  if (importEsm.checkSync()) {
    // Please make sure your engine knows `import()` syntax
    return import(url)
  }

  try {
    return await importEsm(url)
  } catch (error) {
    // Runs only if we don't know support result yet
    handleNonSupportedError(error)
  }
}

Known Issue(s)

• On Node.js@<=0.10, you need polyfill Promise yourself.
• ESM support in Node.js is experimental, though enabled by default in Node.js 13. You will see messages like ExperimentalWarning: The ESM module loader is experimental in console. These are emitted by Node.js, not from this module.