Package Exports

jest-to-log
jest-to-log/src/index.js

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (jest-to-log) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

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