The `before()` Hook in Mocha
By default, Mocha's BDD interface adds a global before() function that registers a function Mocha will run before any tests.
before() is similar to beforeEach(); the key difference is that before() will run at most once, beforeEach() will run once before every test.
// Prints "Running before!"
before(function() {
console.log('Running before!');
});
it('test1', function() {});
it('test2', function() {});before() is typically used to set up any logic that is too slow to execute before every test, like establishing a database connection or starting an HTTP server.
You can use beforeEach() instead of before() in most cases.
With describe()
You can also call before() in a describe().
That tells Mocha to run the before() function before any tests in the describe().
// Prints "Running before!"
describe('suite', function() {
before(function() {
console.log('Running before!');
});
it('test1', function() {});
it('test2', function() {});
});Mocha is smart enough to skip the before() hook if there's no tests to run in your describe().
For example, if you run a single test using .only(), the before() hook in the following example will not run.
// Does **not** print "Running before!"
describe('suite', function() {
before(function() {
console.log('Running before!');
});
it('test1', function() {});
it('test2', function() {});
});
it.only('test3', function() {});Working Around Linters
By default, linters like ESLint complain that before() is not defined.
There are a couple of workarounds.
First, you can explicitly import before() from Mocha:
const { before } = require('mocha');Or, you can set the ESLint Mocha environment in a .eslintrc.js in your test folder as follows.
For example, Mongoose uses this approach to avoid having to explicitly import Mocha hooks.
// .eslintrc.js
module.exports = {
env: {
mocha: true
}
};