# Javascript Array.filter

We will discuss ***Polyfill for Array.***filter***().***

### About

The Javascript filter takes a function as a parameter and returns a new array with items that satisfy the given function .i.e return true when the item is passed to a given function.

```javascript
const arr = [0,2,3,4]

console.log(arr.filter( item => item%2))
console.log(arr)
```

In the above function only `3%2` returns `1` which JS takes as true and the rest of all returns `0` which is false so the output is `[3]`

### Polyfill

1. Return new array
    
2. Check if the element is an array
    
3. Return all the items satisfying the given function.
    

```javascript
Array.prototype.customFilter = function(filterFn){
    let output = []
    if(!Array.isArray(this)){
        return Exception(`${this}.customFilter is not a function`)
    }
    this.forEach((item)=>{
        if(filterFn(item)){
            output.push(item)
        }
    })
    return output
}
```

Here we are looping the given array and checking if the item satisfies the given function. If yes then add it to the output array otherwise, ignore.

```javascript
// Sample output
console.log(arr.customFilter( item => item%2))
// [ 3, 1.2, -1, 5 ]
```

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