made my own forEach function in javascript, but it's returning undefined

There's something about this line:

mapped.push(myForEach(arr, cb));

You're just adding an 'undefined' value (that's what the 'myForEach' function returns) to an empty array.

The callback to map and forEach is "what should I do with each element" - and you're not doing the same thing in myMap's each element as in myForEach's each element.

Inside myMap, you want to do this: For each element of arr, call the myMap's callback, then push the result into the result array. (the emphasized part is the myForEach callback you need to use every time for myMap to function).

Meanwhile, if you want to map [1, 2, 3] to [0, 2, 6], the myMap callback is saying multiply each element with its index and return that value. So you absolutely need two different callbacks.

In short,

myForEach(arr, function(element, i, ary) { mapped.push(cb(element, i ary)); });

The .push() is currently receiving its value directly from myForEach(), which doesn't contain a return statement to provide a value (so, it returns undefined by default).

Being in between the cb and .push(), myForEach() is receiving the results of each cb() and discarding them rather than providing them to myMap().

To avoid that, you'll want move the steps so the .push() and cb() are immediately tied to each other:

mapped.push(cb(elem, i, arr));

You can still use myForEach(), moving it instead around this to perform the iteration, so each elem can be processed by cb and pushed to mapped:

function myMap(arr, cb) {
  var mapped = [];

  myForEach(arr, function (elem, i, arr) {
    mapped.push(cb(elem, i, arr));
  });

  return mapped;
}

The terminology of the map-callback paradigm can be a bit confusing at first.

The map method (e.g. Array.map()) is not the mapping function/callback. It's a loop that applies a callback function to each item of an array.

The callback does the mapping - it maps (or translates) an array item into another value.

So, your callback() function should be taking a single array item (not an array) as an argument, and returning an array item. The map function calls the callback for each item in the array, and assembles the result into a new array. (Alternately, it could return an object, or alter the original array in place).

For example:

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

mycopy = function(element) {
    return element;
}

Then you can use map to duplicate an instance of MyArray:

var oldarray = [...];

var newarray = mymap(oldarray, mycopy);

The point of all this is that you can easily replace mycopy() with something more useful, including functions defined elsewhere or inline anonymous functions:

var positives = mymap(oldarray, Math.abs);

Here is your issue.

console.log(myMap([1, 2, 3], function(ele, i, arr){
  return ele * i;
}));

In the second parameter you are trying to pass an anonymous function. You cannot do this this way due to the hoisting process. Plus there aren't any passed to to this function before it gets executed so it returns undefined.