CODEWITHSUNDEEP
javascript
weima
weima

Reputation: 4902

javascript variable access error

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.

Upvotes: 2

Views: 110

Answers (2)

Doug
Doug

Reputation: 3312

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
},

Upvotes: 3

Scott Sauyet
Scott Sauyet

Reputation: 50797

This is setting a global variable _position:

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

It is not the variable on your pred object.

When you access it here:

run: function () {
    return function (c_) {
        return make_cps(_position, c_);
    }(ret_) == 2;
}

you're using that global variable.

If you want to use instance properties, you're going to need to start using this:

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


run: function () {
    var pred = this;
    return function (c_) {
        return make_cps(pred._position, c_);
    }(ret_) == 2;
}

Upvotes: 0

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