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• Mocking a function in the module

  When testing the doSomething1() function, If the doSomething1() function in ctrl.js module invokes doSomething2() in the same module, you can do as follows.

  • ctrl.js
    

    exports.doSomething1 = (a) => {
        ...
        const result = exports.doSomething2();
    };
    

  • ctrl.test.js

    There is jest.fn(), but if you use jest.spyOn(), you can even check the existence of a function.
    

    const ctrl = require('./ctrl');
    
    describe('ctrl', () => {
        afterEach(() => {
            /* 
             * Initialize to the original module 
             * so that the mock function created using spyOn 
             * does not affect other tests.
            */
            jest.restoreAllMocks();
        });
    
        describe('doSomething1', () => {
            test('...', () => {
                const doSomething2Mock = jest
                    .spyOn(ctrl, 'doSomething2')
                    .mockReturnedValue(1);
                const ret = ctrl.doSomething1();
                ...
            });
        });
    });
    

• Mocking a variable in the module

  If you are mocking the PREFIX variable in module1.js, you can do it as follows.

  • module1.js
    

    exports.PREFIX = 'pre';
    exports.doSomething = a => `${exports.PREFIX}_${a}`;
    

  • module1.test.js
    

    describe('module', () => {
        describe('doSomething', () => {
            let oriPrefix;
    
            beforeAll(() => {
                oriPrefix = module1.PREFIX;
            });
    
            afterAll(() => {
                module1.PREFIX = oriPrefix;
            });
    
            test('test', () => {
                module1.PREFIX = '1';
                ...
            });
        });
    });
    

• Testing a async function

  When testing the doSomething() function in ctrl.js that includes the callback function, do as follows.

  • ctrl.js
    

    exports.doSomething = async () => {...};
    

  • ctrl.test.js
    

    const ctrl = require('./ctrl');
    
    describe('ctrl', () => {
        describe('doSomething', () => {
            test('...', async () => {
                const result = await ctrl.doSomething();
                ...
            });
        });
    });
    

• Testing a callback function(res.json(), ...)

  • ctrl.js
    

    exports.doSomething = (e) => {
        e.on('stop', () => {
            ...
        });
    };
    

  • ctrl.test.js
    

    const ctrl = require('./ctrl');
    
    describe('ctrl', () => {
        describe('doSomething', () => {
            test('...', done => {
                const e = {
                    on: jest.fn(() => {
                        ...
                        done();
                    });
                };
                ctrl.doSomething(e);
            });
        });
    });
    

• Testing throw new Error()

  When checking whether the doSomething() function in ctrl.js throws an error, do as follows.

  • ctrl.js
    

    exports.doSomething = () => {
        throw new Error('?');
    };
    

  • ctrl.test.js
    

    const ctrl = require('./ctrl');
    
    describe('ctrl', () => {
        describe('doSomething', () => {
            test('...', () => {
                expect(() => ctrl.doSomething()).toThrow(Error);
            });
        });
    });
    

  If doSomething() function is asynchronous, you can check whether an error is thrown as follows.

  • ctrl.js
    

    exports.doSomething = async () => {
        ...
        throw Error('?');
    };
    

  • ctrl.test.js
    

    const ctrl = require('./ctrl');
    
    describe('ctrl', () => {
        describe('doSomething', () => {
            test('...', async () => {
                await expect(ctrl.doSomething()).rejects.toThrow(Error);
            });
        });
    });