Stability: 2 - Stable
In the Node.js module system, each file is treated as a separate module. For example, consider a file named foo.js:
const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);
On the first line, foo.js loads the module circle.js that is in the same directory as foo.js.
Here are the contents of circle.js:
const { PI } = Math;
exports.area = (r) => PI * r ** 2;
exports.circumference = (r) => 2 * PI * r;
The module circle.js has exported the functions area() and circumference(). Functions and objects are added to the root of a module by specifying additional properties on the special exports object.
Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable PI is private to circle.js.
The module.exports property can be assigned a new value (such as a function or object).
Below, bar.js makes use of the square module, which exports a Square class:
const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);
The square module is defined in square.js:
// Assigning to exports will not modify module, must use module.exports
module.exports = class Square {
constructor(width) {
this.width = width;
}
area() {
return this.width ** 2;
}
};
The module system is implemented in the require('module') module.
When a file is run directly from Node.js, require.main is set to its module. That means that it is possible to determine whether a file has been run directly by testing require.main === module.
For a file foo.js, this will be true if run via node foo.js, but false if run by require('./foo').
Because module provides a filename property (normally equivalent to __filename), the entry point of the current application can be obtained by checking require.main.filename.