CODEWITHSUNDEEP
javascriptfunctioncallback
BranTheBroken
BranTheBroken

Reputation: 55

Why elements in array dont change after using callback function?

I have some problems with understanding how callback function should work. I wanted to do a function that works just like map method. Even though I don't recive any errors, elements in array don't change. Could you point what am I doing wrong?

function multiplyFn(x) {
    return x * 2;
}
const exampleArray = [1, 2, 3, 4, 5, 6, 8];

function mapFn(array, callback) {
    for (const el of array) {
        callback(el);
        console.log(el)
    }
    console.log(array)
    return array;
}

mapFn(exampleArray, multiplyFn);

Upvotes: 1

Views: 132

Answers (4)

Sunday Cletus
Sunday Cletus

Reputation: 13

If you want to implement your own map function, you can do it without using any of the in-built array methods

//Define your map method in the Array prototype object

Array.prototype.mymap = function(callback) {
    const res = [];
    for (let index = 0; index < this.length; index++) {
        res[index] = callback(this[index], index, this);
    }
    return res;
}
const exampleArray = [1, 2, 3, 4, 5, 6, 8];

//Let's test with two examples

function multiplyFn(x) {
    return x * 2;
}
const first = exampleArray.mymap(multiplyFn)
console.log(first)

const second = exampleArray.mymap((el, index, array) => ({el, index}))
console.log(second)

Upvotes: 0

Zahid Hasan
Zahid Hasan

Reputation: 622

if you want to change the original array you can update the index of the array arguments that you pass in function.

function mapFn(array, callback) {
    array.forEach(function(element,index){
      let val= callback(element)
      array[index] = val
    });
    return array;
}

Upvotes: 1

charlietfl
charlietfl

Reputation: 171690

Note you can do this simply using Array#map() which will not update original but return a new array

function multiplyFn(x) {
    return x * 2;
}
const exampleArray = [1, 2, 3, 4, 5, 6, 8];

function mapFn(array, callback) {    
    return array.map(callback);
}

console.log(mapFn(exampleArray, multiplyFn));

Upvotes: 0

Majed Badawi
Majed Badawi

Reputation: 28424

You need to update the elements by the returned values:

function multiplyFn(x) {
    return x * 2;
}
let exampleArray = [1, 2, 3, 4, 5, 6, 8];

function mapFn(array, callback) {
    for (let i = 0; i < array.length; i++) {
        let el = array[i];
        array[i] = callback(el);
    }
    return array;
}

mapFn(exampleArray, multiplyFn);

Upvotes: 0

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