了解jQuery中的callback.call函数

Question

Please go through the below code. I am not able to exactly what's calllback.call is doing.

Also, I am not able to know what is the difference between this and this[i] and how if(callback.call(this, this[i])) is evaluated to true or false.

Array.prototype.each = function(callback) {
  var i = 0;
  while (i < this.length) {
    callback.call(this, this[i]);
    i++;
  }
  return this;
};

Array.prototype.map = function(callback) {
var i = this.length;
var found = []
while (i--) {
  if(callback.call(this, this[i])) {
    found.push(this[i]);
  }
}
return found;
};

The functions are called below:

Array.each(function(value){
...
})

Array.map(function(value){
...
})

Answer 1

I am not able to exactly what's calllback.call is doing.

Function.prototype.call lets you invoke a function with an explicit value for this

callback.call(this, this[i]);

is the same as

callback(this[i])

except that inside the callback the value of this is the set to be the same as it was in the current context.

---

Also am not able to know what is the difference between this and this[i].

In this context, this is the current array. So this means the whole array, this[i] gets the i'th element of the array.

---

The .each function you have will loop through an array and call a function for each of them. This is similar to javascript's built in Array.prototype.forEach .

The .map is named as though it were a polyfill for Array.protoype.map but is actually doing a .filter operation. Strange.