Function(callback)

Array.prototype.myFilter = function(callback){
  var newArray = [];
  // Add your code below this line
  this.forEach(function(x) {
    if (callback(x) == true) {
      newArray.push(x);
    }
  })
  // Add your code above this line
  return newArray;

};

what does callback mean?
if i change callback to another name ,it also works,so how i understand it

Welcome to The freeCodeCamp Forum — thanks for contributing!

It’s just a function. You can pass functions around the same as any other value in JS. They only execute if you call them (someFuntion vs someFunction()), so you’re just passing a reference to the function. So you can call it whatever you want when you define myFilter, the same as any other argument. It’s just normally called what it is generally known as, a callback (cb, fn, fun are other common names)

With myFilter, you want be able to pass a function to it. That function needs to take a single argument, and return true or false. Then you loop over your array and run the function for each element, pushing all the values that return true into a new array, which you return.

So you don’t need to know what that function is in advance, just that it takes a single value and returns true or false - it could be:

function greaterThan2 (x) { return x > 2; }

or

function notA (x) { return x !== 'a'; }

So then you can do

[1,2,3,4].myFilter(greaterThan2) // returns [3,4]
['a', 'b', 'c'].myFIlter(notA) // returns ['b', 'c']

Or you can pass the callback function right in, same thing:

[1,2,3,4].myFilter(function greaterThan2 (x) { return x > 2; })

Or more tersely (again, exact same thing):

[1,2,3,4].myFIlter(x => x > 2);

That function is just a parameter of the myFilter function, and you can call parameters anything you like

I found this article helpful to learn more about callbacks - https://codeburst.io/javascript-what-the-heck-is-a-callback-aba4da2deced

Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’.