1 · Get comfortable · 18 MIN

Understand imports and explicit exports

A module exposes only the values it deliberately exports.

An import declares where a dependency comes from. The node: prefix identifies a built-in Node module rather than an installed package. This example imports strict assertions to check a small function before printing its result. Exporting a function makes it available to other modules without adding it to a global object. You can later move total into a separate totals.mjs file and import it with a relative path including the extension. Modules help separate reusable calculation from terminal or HTTP concerns.

Read the built-in import before the function. Trace reduce with [2,3] starting from zero. Temporarily make the expected value wrong and read the assertion error. Restore the assertion before adding another test.

Before you start

Install Node.js 22.12+ (or a supported newer LTS). Create a folder named codingneed-node. Open a terminal in that folder. Save ONE lesson example as lesson.mjs and run node lesson.mjs. No npm dependencies are required. HTTP lessons keep running until you press Ctrl+C; stop the previous server before starting another.

File for this example: lesson.mjs

New words, explained

module: A file with its own scope and explicit imports and exports.
export: Make a binding available to another module.
assertion: A check that fails when an expected condition is false.

Follow the example step by step

Read the built-in import before the function.
Trace reduce with [2,3] starting from zero.
Temporarily make the expected value wrong and read the assertion error.
Restore the assertion before adding another test.

You are ready to move on when: The empty array returns zero. The negative-number assertion passes. Explain the difference between node: and a ./ relative import.

Read the example

import assert from 'node:assert/strict';
export function total(values) {
  return values.reduce((sum, value) => sum + value, 0);
}
assert.equal(total([2, 3]), 5);
assert.equal(total([]), 0);
console.log('Total:', total([2, 3]));

Check the expected output

Total: 5. If an assertion fails, Node reports the mismatch and exits with a failure.

Your challenge

Add a test for negative numbers. Optionally move the exported function into totals.mjs and import {total} from ./totals.mjs in lesson.mjs.

Solution cost: Discuss the operations in this small example; rendering and I/O costs depend on the host. time · Proportional to the example’s retained data. space

Common trap

Exporting a function does not call it. Importing a file can execute its top-level statements.

Study the project implementation

import assert from 'node:assert/strict';
export function total(values) {
  return values.reduce((sum, value) => sum + value, 0);
}
assert.equal(total([2, 3]), 5);
assert.equal(total([]), 0);
console.log('Total:', total([2, 3]));

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