Manual mock of object in Jest

Question

I'm trying to figure out manual mocks in Jest. I think this should be easy...but it isn't.

My project directories are like

__tests__
- user-test.js
models
- user.js
- __mocks__
  - user.js
node_modules
- ...
package.json

Both models/user.js and models/__mocks__/user.js have the same code:

module.exports = {
    create(username, password) {
        return new Promise(function(resolve, reject) {
            setTimeout(function() {
                resolve({"username":username, "password": password});
            }, 100);
        });
    }
}

The file __tests__/user-test.js looks like this:

test('user creation works', () => {
    const user = require('../models/user');
    //const user = jest.mock('../models/user');
    return user.create("mitchell", "boo");
});

This works fine, tests pass, but when I change it to:

test('user creation works', () => {
    //const user = require('../models/user');
    const user = jest.mock('../models/user');
    return user.create("mitchell", "boo");
});

It doesn't work, and spits out:

 FAIL  __tests__\user-test.js
  ● user creation works

    TypeError: user.create is not a function

      at Object.<anonymous>.test (__tests__\user-test.js:4:17)
      at process._tickCallback (internal\process\next_tick.js:103:7)

turiyag · Accepted Answer · 2017-01-09 00:06:11Z

27

Ah! Figured it out! My folder structure is fine. Turns out I had a misunderstanding as to what "jest.mock" actually did. "jest.mock" changes the behavior of the node "require" function to use the mock, it doesn't do a require itself.

My test should have looked like:

jest.mock('../models/user');
test('user creation works', () => {
    const user = require('../models/user');
    return user.create("mitchell", "boo");
});

answered Jan 9, 2017 at 0:06

turiyag

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1 Comment

kerolos Over a year ago

Jest mock is used to prevent the functions in a module to be called when module is initialized or an object is instantiated, It is made this way to provide a good shield against other software layers that are not the target of the current unit test

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