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Comment by bzalasky | Hacker News Reader

Because the candidate couldn't write a JavaScript function that allows chaining method calls on its result... along the lines of myFunction().setAttr('a').setAnotherAttr('b').toString(). In my experience, candidates either tackle this in 5-10 minutes, get close and finish with some hints in 20-30 minutes, or have no clue.

If it goes well (first scenario), I like to spend the rest of the interview pairing on a task like writing a parse method for a Backbone model that needs to transform a response for a charting library or something like that with Underscore/Lodash.

One time, a candidate started checking emails on his phone during an on-site interview. That candidate wasn't hired.

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Out of curiosity, what is the things you need to do to make that possible?

You just need to "return this;" for the methods whose result need to be chained.

Example:

var a = function(x){ this.x = x+1; return this; }

var b = function(){ this.x *= 2; return this.x; }

a(3).b(); //result = 8

Something along the lines of this:

  function myFunction() {
    var a, b, func;

    func = function () {
      return {
        attrA: function (attr) {
          a = attr;
          return this;
        },
        
        attrB: function (attr) {
          b = attr;
          return this;
        },

        toString: function () {
          console.log([a, b].join(' '));
          return this;
        }
      }
    }

    return func();
  }

This allows us to call myFunction multiple times, set attributes (imagine it were createPerson instead, with a name, age, etc...) and not run into problems with state being shared.

I like this question because it covers closures, this, objects and functions. It also doesn't qualify as a brain teaser, IMO.

> It also doesn't qualify as a brain teaser, IMO.

That's debatable. It requires a piece of unusual thinking ('return this'), and a bit of magic (knowing about the 'this' magic variable), but once you've seen it, the answer's obvious, and it's pretty easy to replicate everywhere.

Understanding 'this' is foundational when it comes to understanding JavaScript, IMO. So, I don't think it's that unusual. Out of curiosity, what's another question that demonstrates a solid understanding of JavaScript as a language?

Yeah, but returning 'this' to let you chain things like that? Definitely a trick. Not as obscure as Duff's device, but a trick, nonetheless.

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