Modules, imports and file names

1. Export one useful rule

Save this source as totals.mjs. The exported function is available to another module; simply defining it does not call it. Module top-level statements still run when imported, so avoid unrelated CLI work there.

reduce carries an accumulated value through the array. Its callback receives the previous total and the current size, then returns their sum. The final argument 0 is the starting total and also makes an empty list return zero.

export function totalPages(sizes) {
  return sizes.reduce((total, size) => total + size, 0);
}

2. Import from the same practice folder

Save the next file as lesson.mjs beside totals.mjs. In Node modules, include ./ for a neighbouring file and include its extension. The path is relative to this source file, while a later relative data-file path is usually relative to the process’s working folder.

import { totalPages } from "./totals.mjs";
console.log(totalPages([2, 3, 1]));

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3. Avoid accidental format changes

This course consistently uses .mjs and import/export. Node also supports CommonJS with require, but you should not casually mix its assumptions into these examples. A package.json type field can change how .js is interpreted; explicit .mjs filenames keep that decision visible for this route.