Question

I have the code snippet below:

var ret_ = function(x){
    return x;
}
var make_cps=function(x,c_){
    return c_(x);
}
var pred = {
    _position: 0,
    setPosition: function (i) {
        _position = i
    },
    getPosition: function () {
        return _position
    },
    _size: 0,
    setSize: function (i) {
        _size = i
    },
    getSize: function () {
        return _size
    },
    _context: null,
    setContext: function (x) {
        _context = x
    },
    run: function () {
        return function (c_) {
            return make_cps(_position, c_);
        }(ret_) == 2;
    }
}    

When I run it like below, it runs correctly:

pred.setPosition(2)  
pred.setSize(10)  
pred.setContext(null)  
var res = pred.run()  
console.log(res)  // Output: true

but if I replace the _position to getPosition() an error occurs as getPosition() is not defined. Also if I change to this.getPosition() it says this doesn’t have a member called getPosition()

var pred = {
    _position: 0,
    setPosition: function (i) {
        _position = i
    },
    getPosition: function () {
        return _position
    },
    _size: 0,
    setSize: function (i) {
        _size = i
    },
    getSize: function () {
        return _size
    },
    _context: null,
    setContext: function (x) {
        _context = x
    },
    run: function () {
        return function (c_) {
            return make_cps(this.getPosition(), c_); // gives Error here
        }(ret_) == 2;
    }
}    

Please someone throw light on this issue.

Answer 1

You’ve lost your context. Where you’ve put this.getPosition(), this will return as the window object.

If you alter the line to read

return make_cps(pred.getPosition(), c_);

It will work successfully.

Alternately, you can change the run function to read

run: function () {
  var that = this;

  return function (c_) {
      return make_cps(that.getPosition(), c_);
  }(ret_) == 2;
}

Edit: Clarification

The reason that _position is still working rather than suffering from the same issue is that you’re not actually setting prev._position at all in your current code.

setPosition: function (i) {
    _position = i
},
getPosition: function () {
    return _position
}

What it’s actually doing there is creating a new global variable called _position and using that instead.

This code should actually read:

setPosition: function (i) {
    this._position = i
},
getPosition: function () {
    return this._position
},