How to use Jest with jsdom to test console.log?

user_2525633 0
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I'm just getting set up with Jest and I've successfully written unit tests that test the DOM. I have a library that types things on the screen, so I'm able to test just fine. In some cases, instead of throwing an error, my library will spit out a console.warn or console.log. Is it possible to use Jest to test that these console messages are happening?

jestjs unit-testing javascript
2 Answers

Suppose you want test a function like this by printing a message:

function sayHello () { console.log('Hello!') }

You can use jest.spyOn function to change how console.log function behaves.

function sayHello () { console.log('Hello!') };
describe('logging "Hello"', () => {
  const log = jest.spyOn(global.console, 'log');
  sayHello();
  it('should print to console', () => {
    expect(log).toHaveBeenCalledWith('Hello!');
  });
});

OR you can redefine console object and add a key with jest.fn value, like this:

describe('sayHello prints "Hello!"', () => {
  const log = jest.fn()
  global.console = { log }
  sayHello()
  expect(log).toHaveBeenCalledWith('Hello!')
}
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