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Node.js ES Modules

Summary: in this tutorial, you’ll learn how to use ES modules in Node.js via the import and export statements.

How to instruct Node.js to treat JavaScript files as ES modules #

Node.js supports two types of modules:

  • CommonJS
  • ES modules (ESM) (supported in Node 14.0.0 or later).

By default, Node.js treats JavaScript files as CommonJS modules. However, you can tell Node.js to treat JavaScript files as ES modules using one of the following techniques:

  • JavaScript files ending with .mjs
  • JavaScript files ending with .js and the nearest parent package.json file contains a top-level field "type" with a value of "module".
  • Passing the argument --eval, or pied to the node command via STDIN with flag --input-type=module. This technique is rarely used in practice but it is available.

Node.js ES module via .mjs files #

First, create a new file called math.mjs and add the following code:

function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

export { add, subtract };

In the math module:

  • First, define the add and subtract functions.
  • Second, export the add and subtract function using the export statement. This is the named export.

Alternatively, you can place the export keyword before the function like this:

export function add(a, b) {
  return a + b;
}

export function subtract(a, b) {
  return a - b;
}

Second, create a new file called app.mjs that uses the math module:

import { add, subtract } from './math.mjs';

let a = 20,
    b = 5;

let result = add(a, b);
console.log(`${a} + ${b} = ${result}`);

result = subtract(a, b);
console.log(`${a} - ${b} = ${result}`);

In the app.mjs file:

  • First, import the add and substract functions from the math module using the import statement.
  • Second, call the add and subtract functions.

Third, run the app.mjs file using the node command:

node app.mjs

You should see the following in the console:

20 + 5 = 25
20 - 5 = 15

In this example, we use the files that end in .mjs, Node.js treats all of them as ES modules.

Set type module in package.json file #

First, rename the math.mjs and app.mjs to math.js and app.js respectively.

Second, create a package.json file with the top-level field type with the value "module":

{
  "type": "module"
}

Third, run the app.js file using the node command:

node app.js

You should see the same output as shown in the first example.

Using the eval command line argument #

The following example runs the node command with the eval argument and the --input-type=module:

node --input-type=module --eval "import { delimiter } from 'path'; console.log(delimiter);"

Output:

;

In this example, we pass the following code to the --eval argument:

import { delimiter } from 'path';console.log(delimiter);

This imports the delimiter from the path module and outputs it to the console.

Likewise, you can use the following command:

echo "import { delimiter } from 'path'; console.log(delimiter);" | node --input-type=module

Summary #

  • Use JavaScript files with .mjs extension to instruct Node.js to treat the JavaScript files as ES modules.
  • Set the "type": "module" field in the package.json to also instruct Node.js to treat JavaScript files as ES modules.

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