Tukukkk

In the following code how do I get the this keyword to bind to Square?

Square.prototype.is = {

  king: function () { this.piece === "K" }

  queen: function () { this.piece === "Q" }

  ...

};

I know I can later on use call/apply but the whole point was to get from

this.isKing()

to

this.is.king()

making it more readable (also grouping the methods together)

this.is.king.apply(this)

seems like a step backwards.

I would suggest using ES6 arrow functions. This is a syntax that is supported by all major browsers now. Using arrow functions allows us to leverage the outer scope's this.

An example using Class:

class Square {

  constructor(){

    this.piece = null;

    this.is = {

      king: () => this.piece === "K",

      queen: () => this.piece === "Q"

    }

  }

};



const square = new Square();

square.piece = 'K';



console.log(square.is.king());

console.log(square.is.queen());

If you're determined to use ES5 syntax:

var Square = function Square() {

  var _this = this;

  this.piece = null;

  this.is = {

    king: function() { return _this.piece === "K" },

    queen: function() { return _this.piece === "Q" }

  };

};





var square = new Square();

square.piece = 'K';



console.log(square.is.king());

console.log(square.is.queen());

Finally, per @Bergi's comment, here's one more approach, using arrow functions and objects, without any Classes.

const Square = function Square() {  

  this.piece = null;

  this.is = {

    king: () => this.piece === "K",

    queen: () => this.piece === "Q"

  };

};





const square = new Square();

square.piece = 'K';



console.log(square.is.king());

console.log(square.is.queen());

I know this does not answer directly to your question, but the is functions that are commonly found in third party libraries usually have the following signature: is(type).

In this fashion, you could write:

square.prototype.is = function(type) {

  switch (String(type).toLowerCase()) {

    case "king": return this.piece === "K";

    case "queen": return this.piece === "Q";

    default: return false;

  }

}

Assuming type would be a String. But it can an Number defined in an Object to act as a Enum.

I hope you won't consider this off-topic as it can be a solution to consider alternative ways :)