How to write a test case for a setInterval() function

Question

I have a timer and assume a specific function will be executed when the counter is counting to 3.

var a_interval_function = function(){
    var counter = 1;
    var interval = setInterval(function(){
        if(counter === 5){
            clearInterval(interval);
        }

        // run the function when the counter is 3
        if(counter === 3){
            a_function_should_be_runned();
        }   

        counter++;
    }, 500);

    return interval;
}

However, I don't know how to establish a valid test case for testing the counter as well as the timing when the function is executed. Does anyone know how to do it? Something like the following:

// and some test case like this
it('a timer test', function(done){
    var interval = a_interval_function();
    expect(a_function_should_be_runned.state).to.equal({
        name: 'runned',
        counter: 3,
        time: 300,
    });
});

Thanks.

Answer 1

Perhaps you can use sinon.useFakeTimers().

For example:

var sinon  = require('sinon');
var expect = require('chai').expect;

var a_function_should_be_runned = sinon.spy();

var a_interval_function = function(){
  var counter = 1;
  var interval = setInterval(function(){
    if(counter === 5){
      clearInterval(interval);
    }

    // run the function when the counter is 3
    if(counter === 3){
      a_function_should_be_runned();
    }   

    counter++;
  }, 500);

  return interval;
}

describe('timer tests', function() {

  before(function() {
    this.clock = sinon.useFakeTimers();
  });

  after(function() {
    this.clock.restore();
  });

  it('a timer test', function() {
    var interval = a_interval_function();

    // At time 0, we don't expect the function to have been called.
    expect(a_function_should_be_runned.called).to.be.false;

    // Advance clock 500ms.
    this.clock.tick(500);
    expect(a_function_should_be_runned.called).to.be.false;

    // Advance clock again (1s since start)
    this.clock.tick(500);
    expect(a_function_should_be_runned.called).to.be.false;

    // Advance clock again (1.5s since start). This should
    // trigger the call to `a_function_should_be_runned`.
    this.clock.tick(500);
    expect(a_function_should_be_runned.called).to.be.true;
  });
});