Mocha Monitor Application Output - node.js

Question

I'm building a logging module for my web app in nodejs. I'd like to be able to test using mocha that my module outputs the correct messages to the terminal. I have been looking around but haven't found any obvious solutions to check this. I have found
process.stdout.on('data', function (){})
but haven't been able to get this to work. does anybody have any advice?

Answer 1

process.stdout is never going to emit 'data' events because it's not a readable stream. You can read all about that in the node stream documentation, if you're curious.
As far as I know, the simplest way to hook or capture process.stdout or process.stderr is to replace process.stdout.write with a function that does what you want. Super hacky, I know, but in a testing scenario you can use before and after hooks to make sure it gets unhooked, so it's more or less harmless. Since it writes to the underlying stream anyway, it's not the end of the world if you don't unhook it anyway.
function captureStream(stream){
var oldWrite = stream.write;
var buf = '';
stream.write = function(chunk, encoding, callback){
buf += chunk.toString(); // chunk is a String or Buffer
oldWrite.apply(stream, arguments);
}
return {
unhook: function unhook(){
stream.write = oldWrite;
},
captured: function(){
return buf;
}
};
}
You can use it in mocha tests like this:
describe('console.log', function(){
var hook;
beforeEach(function(){
hook = captureStream(process.stdout);
});
afterEach(function(){
hook.unhook();
});
it('prints the argument', function(){
console.log('hi');
assert.equal(hook.captured(),'hi\n');
});
});
Here's a caveat: mocha reporters print to the standard output. They do not, as far as I know, do so while example (it('...',function(){})) functions are running, but you may run into trouble if your example functions are asynchronous. I'll see if I can find more out about this.

Answer 2

I've attempted jjm's answer and had problems which I suspect was due to my programs async behaviour.
I found a solution via a cli on github that uses the sinon library.
An example code to test:
/* jshint node:true */
module.exports = Test1;
function Test1(options) {
options = options || {};
}
Test1.prototype.executeSync = function() {
console.log("ABC");
console.log("123");
console.log("CBA");
console.log("321");
};
Test1.prototype.executeASync = function(time, callback) {
setTimeout(function() {
console.log("ABC");
console.log("123");
console.log("CBA");
console.log("321");
callback();
}, time);
};
And the mocha tests:
/* jshint node:true */
/* global describe:true, it:true, beforeEach:true, afterEach:true, expect:true */
var assert = require('chai').assert;
var expect = require('chai').expect;
var sinon = require("sinon");
var Test1 = require("../test");
var test1 = null;
describe("test1", function() {
beforeEach(function() {
sinon.stub(console, "log").returns(void 0);
sinon.stub(console, "error").returns(void 0);
test1 = new Test1();
});
afterEach(function() {
console.log.restore();
console.error.restore();
});
describe("executeSync", function() {
it("should output correctly", function() {
test1.executeSync();
assert.isTrue(console.log.called, "log should have been called.");
assert.equal(console.log.callCount, 4);
assert.isFalse(console.log.calledOnce);
expect(console.log.getCall(0).args[0]).to.equal("ABC");
expect(console.log.getCall(1).args[0]).to.equal("123");
expect(console.log.args[2][0]).to.equal("CBA");
expect(console.log.args[3][0]).to.equal("321");
});
});
describe("executeASync", function() {
it("should output correctly", function(done) {
test1.executeASync(100, function() {
assert.isTrue(console.log.called, "log should have been called.");
assert.equal(console.log.callCount, 4);
assert.isFalse(console.log.calledOnce);
expect(console.log.getCall(0).args[0]).to.equal("ABC");
expect(console.log.getCall(1).args[0]).to.equal("123");
expect(console.log.args[2][0]).to.equal("CBA");
expect(console.log.args[3][0]).to.equal("321");
done();
});
});
});
});
I'm providing the above as it demonstrates working with async calls, it deals with both console and error output and the method of inspection is of more use.
I should note that I've provided two methods of obtaining what was passed to the console, console.log.getCall(0).args[0] and console.log.args[0][0]. The first param is the line written to the console. Feel free to use what you think is appropriate.

Answer 3

Two other libraries that help with this are test-console and intercept-stdout I haven't used intercept-stdout, but here's how you can do it with test-console.
var myAsync = require('my-async');
var stdout = require('test-console').stdout;
describe('myAsync', function() {
it('outputs something', function(done) {
var inspect = stdout.inspect();
myAsync().then(function() {
inspect.restore();
assert.ok(inspect.output.length > 0);
done();
});
});
});
Note: You must use Mocha's async api. No calling done() will swallow mocha's test messaging.