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JavaScript

Posted by Mayur Pande
Mayur Pande
Courses Plus Student 11,711 Points

Javascript filter function

I am having difficulties fully grasping the filter fn in JS.

I have this code;

function destroyer(arr) {
  // Remove all the values
  var args = Array.prototype.slice.call(arguments);
  //console.log(args[2]);
  //console.log(arr);

  function seekAndDestroy(val,index,ar){

    for(var i = 1; i< args.length;i++){
        console.log(args[i]);
    }

  }

  var result = arr.filter(seekAndDestroy);
  //console.log(result);

}

destroyer([1, 2, 3, 1, 2, 3], 2, 3);

This will give the result

2
3
2
3
2
3
2
3
2
3
2
3

My question here is why does it print it out six time, surely it should only count to args.length which is two

1 Answer

Thomas Nilsen
Thomas Nilsen
14,957 Points

There is no need to loop through anything. The filter method does that for you.

All you really need to give the filter function is another function that returns a boolean, which determines wether or not the value will be filtered out. 

function onlyEvenNumbers(number) {
    return number % 2 == 0;
}


var result = [1, 2, 3, 4, 5, 6].filter(onlyEvenNumbers);
console.log(result); // prints out [2, 4, 6]

Here is a simplified version of a filter function:

Array.prototype.customFilter = function(callback) {
    var result = [];
    for (var i = 0; i < this.length; i++) {
        if(callback(this[i])) {
            result.push(this[i]);
        }
    }

    return result;
}

function onlyEvenNumbers(number) {
    return number % 2 == 0;
}


var result = [1, 2, 3, 4, 5, 6].customFilter(onlyEvenNumbers);
console.log(result); //Still prints out [2, 4, 6]
Mayur Pande
Mayur Pande
Courses Plus Student 11,711 Points

thanks for the answer, I mostly understand it now, but how does it work if returning boolean expressions such as true or false?

Mayur Pande
Mayur Pande
Courses Plus Student 11,711 Points

actually don't worry got it now! thanks