Callback function not executing

The array, that is returned should be [1,2,3], however instead [3,4,5] is being logged to the console. Can somebody explain to me why?

[1,2,3]

[3,4,5]

function forEach(array, callback) {
var newA =
for (var i = 0; i < array.length; i++) {
newA.push(callback(array[i]));
}
}

function map(array, callback) {
forEach(array, callback)
return array
}

console.log(map([3, 4, 5], n => n - 2));

array

forEach

callback

array[i]

return newA;

forEach

return forEach(array,callback);

map

2 Answers
2

Your map() function requires that forEach() modifies the array in place, rather than creating a new array. So it should be:

map()

forEach()

function forEach(array, callback) {
for (var i = 0; i < array.length; i++) {
array[i] = callback(array[i]);
}
}

function map(array, callback) {
forEach(array, callback);
return array;
}

console.log(map([3, 4, 5], n => n - 2));

If map() isn't supposed to modify its argument, you could make a copy of the array before calling forEach().

map()

forEach()

function forEach(array, callback) {
for (var i = 0; i < array.length; i++) {
array[i] = callback(array[i]);
}
}

function map(array, callback) {
array = array.slice();
forEach(array, callback);
return array;
}

var testArray = [3, 4, 5];
console.log(map(testArray, n => n - 2));
console.log(testArray);

Callback Function not executing

Yes, it does, you just need to return the new array properly

Stack snippet

function forEach(array, callback) {
var newA = ;
for (var i = 0; i < array.length; i++) {
newA.push(callback(array[i]));
}
return newA; // added
}

function map(array, callback) {
return forEach(array, callback); // changed
}
console.log(map([3, 4, 5], n => n - 2));

Or deep clone the array and clear it, like this

array

function forEach(array, callback) {
var newA = JSON.parse(JSON.stringify(array)); // deep clone
array.length = 0; // clear the array
for (var i = 0; i < newA.length; i++) {
array.push(callback(newA[i]));
}
}

function map(array, callback) {
forEach(array, callback);
return array
}
console.log(map([3, 4, 5], n => n - 2));

or this

function forEach(array, callback) {
var newA = array.slice(); // shallow copy
array.length = 0; // clear the array
for (var i = 0; i < newA.length; i++) {
array.push(callback(newA[i]));
}
}

function map(array, callback) {
forEach(array, callback);
return array
}
console.log(map([3, 4, 5], n => n - 2));

map()

foreach()

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