systems-obscure/posts/suppressing-logs-errors-jest.md

title	slug	date	tags
Suppressing logs and errors in Jest	/suppressing-logs-errors-jest/	2024-09-10	javascript unit-testing

It annoys me when I am working on a project and a previous developer has left logs and/or thrown errors in their unit tests. To be clear: I mean when the developer is testing that an error is thrown in the right circumstances, not a test failure arising from regression. The former makes the latter harder to detect by polluting the output.

This can be so easily prevented.

Take the following function:

function sillyFunction(int) {
  console.info(`Now handling ${int}`)
  if (int > 2) throw new Error(`Error: int ${int} is greater than two`)
  else return
}

To avoid pointlessly logging to the console and to confirm the error without actually throwing it:

import { jest } from "@jest/globals"

describe("sillyFunction", () => {
  beforeEach(() => {
    jest.spyOn(console, "info").mockImplementation(() => {})
  })

  afterEach(() => {
    console.info.mockRestore()
  })

  it("throws error if `int` is less than 2", () => {
    expect(() => {
      sillyFunction(3)
    }).toThrow("Error: int 3 is greater than two")
  })
})

The spyOn method silences the output of console.error by returning nothing. If we wish, we can still confirm that the log is acting as expected during the runtime of the test by again using a spy:

const consoleInfoSpy = jest.spyOn(console, "info")
expect(consoleInfoSpy).toHaveBeenCalledWith("Now handling x")

The toThrow method catches the error before it hits the console and allows us to interrogate it. If it was an asynchronous function under test, we would need to use the matcher rejects.toThrow. This waits for the promise to resolve before checking if it has been rejected.