CODEWITHSUNDEEP
javascriptobject
dthree
dthree

Reputation: 20770

JS - Overwrite 'this' in function

I am trying to extend the Array prototype:

Array.prototype.rotate = function() {
  var arr = [];
  for (var i = 0; i < this[0].length; ++i) {
    arr[i] = [];
    for (var j = 0; j < this.length; ++j) {
      arr[i].push(this[j][i])
    }
  }
  this = arr;
  return arr;
}

Totally dandy, until this = arr. That bombs out.

How do I re-assign the this property of a prototypal function? I want the hells to do with the previous array configuration.

EDIT

Why am I doing it this way? I want it to behave like other array functions. For example, this works:

myArray.pop();

I don't need to do this:

myArray = myArray.pop();

ANOTHER EDIT

I did this to solve it, but it seems stupid:

Array.prototype.rotate = function() 
{
  var arr = [];
  var len = this[0].length;
  for (var i = 0; i < len; ++i) {
    arr[i] = [];
    for (var j = 0; j < this.length; ++j) {
      arr[i].push(this[j][i])
    }
  }
  for (var i = 0; i < this.length; ++i) {
    delete this[i];
  }
  for (var i = 0; i < arr.length; ++i) {
    this[i] = arr[i];
  }
  return arr;
}

This would work, but, in an example, when rotating this array:

[[1, 1], [2, 2], [3, 3]]

I would get:

[[1, 2, 3], [1, 2, 3], ]

See that little blank third item? Yeah - that caused me problems.

Upvotes: 5

Views: 2986

Answers (3)

Xeltor
Xeltor

Reputation: 4654

As @Meredith said, if you modify this instead of a new array, you'll have the result you expect. the following test worked for me :

Array.prototype.rotate = function(){
    var a = this[0];
    this[0] = this[1];
    this[1] = a;
};

var test = [1,2];
test.rotate();
console.log(test); // [2,1]

Upvotes: 0

Sean Johnson
Sean Johnson

Reputation: 5607

Although you cannot overwrite this, you can most certainly set the value of the working array to the array you've built out in your function. Something like this:

Array.prototype.rotate = function() {
  var arr = [];
  for (var i = 0; i < this[0].length; ++i) {
    arr[i] = [];
    for (var j = 0; j < this.length; ++j) {
      arr[i].push(this[j][i])
    }
  }
  this.length = 0; //empty the array
  this.push.apply(this, arr); //push all of the elements from arr onto this
  return arr;
}

Upvotes: 9

Meredith
Meredith

Reputation: 840

You can't edit this, but you can edit this[i][j], so copy your temporary array into this with a loop.

Upvotes: 0

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