Colocating tests is putting your test next to its file.

Why colocate tests? Because colocating provides:

• Better visibility
• Simpler mental models
• Less context switching

Layout

Example of a directory structure containing colocated tests:

.
└── src
    ├── file.js
    └── file.test.js

Example of a directory structure containing non-colocated tests:

.
├── src
│   └── file.js
└── test
    └── file.test.js

Import

Colocated tests simplifies import paths:

// src/file.test.js
const file = require('./file');

Compared to non-colocation:

// test/file.test.js
const file = require('../test/file');

For small projects, non-colocation is manageable. But for larger projects, the import paths can get unwieldly:

const file = require('../../../test/dir/subdir/file');

Refactor

For colocated tests, the test will pass even if the directory is renamed.

For non-colocated tests, the test will fail if the directory is renamed, which leads to a false negative.

When a feature is removed, it’s simple to delete both the file and the test.

Runners

Jest supports colocation by default.

Mocha requires some configuration:

npx mocha src/**/*.test.js