Why doesn't my filter function work? (Javascript)

The given code, does the filtering part, because any quadratic positive integer is a truthy value, but it does not return the new value.

It looks like, you need two actions, one filtering and changing the filtered values.

In this case you need Array#filter for getting positive interger numbers and Array#map for a quadratic value.

let array = [4, -4, 5.8, 1.6],
    result = array
        .filter(x => x > 0 && Number.isInteger(x))
        .map(x => x * x);

console.log(result);

The reason is already explained in previous answer, you can use reduce if you want to use single function

let arr = [4, -4, 5.8, 1.6];
let newA = arr.reduce(function(accumulator, current) {
  if (current > 0 && Number.isInteger(current)) {
    accumulator.push(current * current);
  }
  return accumulator
}, [])
console.log(newA)

Filter just filters the value and return boolean result.. You can use map as below,

let arr = [4, -4, 5.8, 1.6];
let newA = arr.filter((x) => x > 0 && Number.isInteger(x))
.map((x) => x * x);

console.log(newA);

With reduce method :

let arr = [4, -4, 5.8, 1.6];

let newA = arr.reduce( ( acc, x ) => {
  if ( x > 0 && Number.isInteger(x) ) { acc = acc.concat( [x*x] ) } ;
  return acc;
}, []) ;

console.log(newA);

Not sure about the use case, but looks like to be "get the square of all positive integers in an array". If it is then the below code should work.

let arr3=[4,-4,5.8,1.6];   
let new3 = arr3.filter((x)=>{ 
    return (x>0 && Number.isInteger(x));
}).map(function(y){ 
return y*y;
})

First extract the list of all positive integers and then return the square of each item in the list.