jest-to-log

What

Test log messages with jest

Why?

Grown out of the need to test console.log/error messages or writes to process.stdout/stderr in my cli-facing apps. Note: this is not a full-blown cli-testing tool, but a check for important log messages from your app that's tested with jest.

Usage

require("jest-to-log");
//or
import "jest-to-log";

Example

//in jest test file
require("jest-to-log");

describe("toLog", () => {

  it("should capture log messages", () => {

    function testFn() {

      console.info("Jello", 1);
      console.log("Jello", 2);
      console.debug("Jello", 3);
    }
    
    const expectedString =
      "Jello 1" + "\n" + "Jello 2" + "\n" + "Jello 3" + "\n";

    expect(testFn).toLog(expectedString);
  });
});

Async Example

When an async function is tested, the expect assertions must be awaited.

//in jest test file (esm)
import("jest-to-log");

describe("process.stdout.write", () => {

  it("should capture process.stdout.write messages", async () => {

    async function asyncTestFn() {

      await new Promise((resolve) => {

        setTimeout(() => {
          process.stdout.write(`Jello 123\n`);
          process.stdout.write(`Jello 456\n`);

          resolve();

        }, 500);
      });
    }

    const expectedString = "Jello 123" + "\n" + "Jello 456" + "\n";

    await expect(asyncTestFn).toLog(expectedString);
  });
});

Caveat

• When using with typescript (i.e., ts-jest and @types/jest), it's advised to explicitly import the expect from @jest/globals to avoid type conflicts:

//test.ts

import { expect } from "@jest/globals";

// ...your test

• Do NOT set the injectGlobals jest option to false, or else a ReferenceError will be thrown. This is because the matchers are extended onto the expect object without importing it from @jest/globals.

Matchers

Matcher Name	Description
toLog	Checks if a function logs a message via console.log(), console.info() or console.debug().
toLogStdout	Checks if a function explicitly writes to process.stdout. Cannot be used to capture console.log/info/debug calls due to the custom console implementation of jest.
toLogStderr	Checks if a function explicitly write to process.stderr. Cannot be used to capture console.error/warn calls due to the custom console implementation of jest.
toLogErrorOrWarn	Checks if a function logs an error or warning via console.error() or console.warn(), respectively.

Installation

$ npm install jest-to-log

Requirements

• Latest versions of Jest recommended

Dependencies

• 0 javascript/nodejs dependency!

Test

$ npm test