Filter.md

filter() method

The filter() method in JavaScript creates a new array with all elements that pass a test implemented by the provided function.

Here's how it works:

1. It creates a new array with all elements that pass a test (provided as a function).
2. The provided function is called with three arguments: the current element, the index of the current element, and the array filter() was called upon.
3. It does not mutate the original array.

Here's a simple example:

const numbers = [1, 2, 3, 4, 5];

const evenNumbers = numbers.filter(function(number) {
    return number % 2 === 0;
});

console.log(evenNumbers); // Output: [2, 4]

In this example, filter() is called on the numbers array and returns a new array evenNumbers where each number is even.

Syntax

The syntax for the filter() method in JavaScript is as follows:

array.filter(callbackFunc(currentValue, index, arr), thisValue)

Here's what each parameter means:

• callbackFunc(currentValue, index, arr): A function to be called for each element in the array. This function accepts three arguments:
  • currentValue: The current element being processed in the array.
  • index (optional): The index of the current element being processed in the array.
  • arr (optional): The array filter was called upon.
• thisValue (optional): A value to use as this when executing the callback function.

The filter() method returns a new array with all elements that pass the test implemented by the provided function.

Polyfill

const numbers = [1, 2, 3, 4, 5];

Array.prototype.filter = function(callbackFunc){
    const newArr = []
    for(let i=0; i<this.length; i++){
       if(callbackFunc(this[i])){
          newArr.push(callbackFunc(this[i]))
       }
    }
    return newArr;
}

console.log(numbers.map(evenNumbers)) // Output: [2, 4]