I need to test what console.log() printing. For example:

console.log("one %s three", two) ==> "one two three"

How can I get this final message ("one two three") using Jest?

P.s. It is important to check printing, not input parameters. Is it possible?

I need to test what console.log() printing. For example:

console.log("one %s three", two) ==> "one two three"

How can I get this final message ("one two three") using Jest?

P.s. It is important to check printing, not input parameters. Is it possible?

• javascript
• jestjs

• instead of testing what console.log prints.; test the variable "two" – boop_the_snoot Commented Jun 23, 2022 at 18:33
• Why you want to test console.log, why not just test the value ? console.log is already well tested – Code Maniac Commented Jun 23, 2022 at 18:33
• @Code Maniac Someone could have console.log = function(str) {console.error("something else")} – IT goldman Commented Jun 23, 2022 at 18:36
• I need to test output of winston's wrapper. I believe it will be same principal as console.log testing. – Nikita Polevoy Commented Jun 23, 2022 at 18:38

1 Answer 1

You may not want to test console.log internal functionality (ensure that "one two three" gets printed) as that's not something you developed. The idea of unit tests is testing your own code and not external functionality, you just assume its gonna work as you would expect it to do. If it suddenly didn't do what you were expecting it's not like you are going to fix console.log inner code/functionality.

However what you could validate is that console.log gets called with the parameters you are expecting, which in this case you could achieve by spying on the log method:

const logSpy = jest.spyOn(console, 'log');

// I assume two is a variable? If not then don't forget to double quote like "two"
expect(logSpy).toHaveBeenCalledWith("one % three", two);