How to mock the Node.js child_process spawn function?

For anyone who still has problems with this particular problem and for some reason, the recommendations in other answers don't help, I was able to get it to work with proxyrequire (https://github.com/thlorenz/proxyquire) by replacing the real child_process spawn with an event emitter that I then used in my tests to mock the emission.

var stdout = new events.EventEmitter();
var stderr = new events.EventEmitter();
var spawn = new events.EventEmitter();
spawn.stderr = stderr;
spawn.stdout = stdout;

var child_process = {
  spawn: () => spawn,
  stdout,
  stderr
};

// proxyrequire replaces the child_process require in the file pathToModule
var moduleToTest = proxyquire("./pathToModule/", {
  'child_process': child_process
});

describe('Actual test', function () {
  var response;

  before(function (done) {
    // your regular method call
    moduleToTest.methodToTest()
    .then(data => {
      response = data;
      done();
    }).catch(err => {
      response = err;
      done();
    });

    // emit your expected response
    child_process.stdout.emit("data", "the success message sent");
    // you could easily use the below to test an error
    // child_process.stderr.emit("data", "the error sent");
  });

  it('test your expectation', function () {
    expect(response).to.equal("the success message or whatever your moduleToTest 
      resolves with");
  });
});

Hope this helps...

you can use sinon.stubs sinon stubs guide

// i like the sandbox, or you can use sinon itself
let sandbox = sinon.sandbox.create();

let spawnEvent = new events.EventEmitter();
spawnEvent.stdout = new events.EventEmitter();

sandbox.stub(child_process, 'spawn').returns(spawnEvent);

// and emit your event
spawnEvent.stdout.emit('data', 'hello world');

console.log(output)  // hello world

I've found the mock-spawn library, which pretty much does what I want. It allows to mock the spawn call and provide expected results back to the calling test.

An example:

var mockSpawn = require('mock-spawn');

var mySpawn = mockSpawn();
require('child_process').spawn = mySpawn;

mySpawn.setDefault(mySpawn.simple(1 /* exit code */, 'hello world' /* stdout */));

More advanced examples can be found on the project page.

Came across this and nwinkler's answer put me on the path. Below is a Mocha, Sinon and Typescript example that wraps the spawn in a promise, resolving if the exit code is a zero, and rejecting otherwise, It gathers up STDOUT/STDERR output, and lets you pipe text in through STDIN. Testing for a failure would be just a matter of testing for the exception.

function spawnAsPromise(cmd: string, args: ReadonlyArray<string> | undefined, options: child_process.SpawnOptions | undefined, input: string | undefined) {
    return new Promise((resolve, reject) => {
        // You could separate STDOUT and STDERR if your heart so desires...
        let output: string = '';  
        const child = child_process.spawn(cmd, args, options);
        child.stdout.on('data', (data) => {
            output += data;
        });
        child.stderr.on('data', (data) => {
            output += data;
        });
        child.on('close', (code) => {
            (code === 0) ? resolve(output) : reject(output);
        });
        child.on('error', (err) => {
            reject(err.toString());
        });

        if(input) {            
            child.stdin.write(input);
            child.stdin.end();
        }
    });
}

// ...

describe("SpawnService", () => {
    it("should run successfully", async() => {
        const sandbox = sinon.createSandbox();
        try {
            const CMD = 'foo';
            const ARGS = ['--bar'];
            const OPTS = { cwd: '/var/fubar' };

            const STDIN_TEXT = 'I typed this!';
            const STDERR_TEXT = 'Some diag stuff...';
            const STDOUT_TEXT = 'Some output stuff...';

            const proc = <child_process.ChildProcess> new events.EventEmitter();
            proc.stdin = new stream.Writable();
            proc.stdout = <stream.Readable> new events.EventEmitter();
            proc.stderr = <stream.Readable> new events.EventEmitter();

            // Stub out child process, returning our fake child process
            sandbox.stub(child_process, 'spawn')
                .returns(proc)    
                .calledOnceWith(CMD, ARGS, OPTS);

            // Stub our expectations with any text we are inputing,
            // you can remove these two lines if not piping in data
            sandbox.stub(proc.stdin, "write").calledOnceWith(STDIN_TEXT);
            sandbox.stub(proc.stdin, "end").calledOnce = true;

            // Launch your process here
            const p = spawnAsPromise(CMD, ARGS, OPTS, STDIN_TEXT);

            // Simulate your program's output
            proc.stderr.emit('data', STDERR_TEXT);
            proc.stdout.emit('data', STDOUT_TEXT);

            // Exit your program, 0 = success, !0 = failure
            proc.emit('close', 0);

            // The close should get rid of the process
            const results = await p;
            assert.equal(results, STDERR_TEXT + STDOUT_TEXT);
        } finally {
            sandbox.restore();
        }
    });
});