Test a console log function

Question

I have a function that prints a string
I want to test that function actually prints the string provided

The way I understand this should work is: 1. store the log in a var 2. assert content stored in the var is toBe what I expect

const log = 'hello2';

let myFunction = (log) => {
    console.log(log);
    };
myFunction(log);

const {myFunction} = require('../function-conversion');

test('Function prints message: "hello"', () => {
    expect(myFunction(log).toBe(console.log("hello")))
});

Function prints message: "hello" (1ms)

Function prints message: "hello"

ReferenceError: log is not defined

at Object.log (tests/function-conversion.test.js:4:20)

console.log ../function-conversion.js:11 hello2

I am not sure how your test should succeed, console.log does not return a value. If you want to test that a function actually prints out something, you have to redirect the standard output stream to a file and check its contents after writing. — collapsar
– collapsar, Commented Jul 19, 2019 at 10:28
btw you have confusion in code between log as params and log as declared outside. should be better if they have different names — duc mai
– duc mai, Commented Jul 19, 2019 at 10:29

Hardik Shah · Accepted Answer · 2019-07-19 11:03:31Z

1

There is an error to below line:

expect(myFunction(log).toBe(console.log("hello")))

While calling function myFunction(log) you should pass any string instead of log variable which is not declared anywhere (into function-conversion.test.js file) like "hello".

I don't have an environment up so couldn't test. But what you can do is:

describe('SearchResultsComponent', () => {
  let spy;
  
  let myFunction = (log) => {
    console.log(log);
  };
  myFunction(log);
  
  beforeAll(async () => {
    spy = jest.fn();
  });

  test('Function prints message: "hello"', () => {
    const log = 'hello';
    spy = jest.spyOn(console, 'log');
    myFunction(log);
    expect(spy).toHaveBeenCalled();
    expect(spy.calls.argsFor(0)).toEqual(['hello']);
  });
});

answered Jul 19, 2019 at 11:03

Hardik Shah

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5 Comments

Kinfol Over a year ago

Thank you for your comment Hardik Shah, it is a very good explanation!

Hardik Shah Over a year ago

@Kinfol, If it helps you, then, please upvote it and you can mark this as an answer. So other can benefit. Thanks!

Kinfol Over a year ago

I am sorry, i can not upvote it. I do not have enough experience yet to do it.

Hardik Shah Over a year ago

No worry. You can click on up icon near by my answer and click on tick mark grey icon to accept it as an answer.

Kinfol Over a year ago

The reason i said it was a good explanation it is because it kinda lead me to the response. Your code has this error in it Cannot read property 'argsFor' of undefined I should have specified that I d not want to mock this test, but I just want to use a simple jest test, to understand how this works. I will explain in my answer.

Kinfol · Accepted Answer · 2019-07-28 18:20:30Z

I pumped up the initial function a bit. In case one should test a console.log message, he should push the string to an array.

let arrayOfMessages = [];
let functionThatPrintsTheArrayOfMessages = () => {
// pushing the messages to the array can be done when invoking the function as in line 19
//  console.log(message.push('hello','hi'));
    for (let i in arrayOfMessages){
    console.log(arrayOfMessages[i])
    };
    return arrayOfMessages;
};
functionThatPrintsTheArrayOfMessages(arrayOfMessages.push('hello', 'hi'));

This is the jest test.

test('Function prints message: "hello", "hi"', () => {
expect(functionThatPrintsTheArrayOfMessages()).toContain('hello')

expect(functionThatPrintsTheArrayOfMessages()).toContain('hi')

expect(functionThatPrintsTheArrayOfMessages()).toEqual(['hello', 'hi'])
});

I would like to know if this is correct and other ways of testing. Involving Jest.

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