jQuery map vs. each

Question

In jQuery, the map and each functions seem to do the same thing. Are there any practical differences between the two? When would you choose to use one instead of the other?

Answer 1

Jquery.map makes more sense when you are doing work on arrays as it performs very well with arrays.

Jquery.each is best used when iterating through selector items. Which is evidenced in that the map function does not use a selector.

$(selector).each(...)

$.map(arr....)

as you can see, map is not intended to be used with selectors.

Answer 2

The each method is meant to be an immutable iterator, where as the map method can be used as an iterator, but is really meant to manipulate the supplied array and return a new array.

Another important thing to note is that the each function returns the original array while the map function returns a new array. If you overuse the return value of the map function you can potentially waste a lot of memory.

For example:

var items = [1,2,3,4];

$.each(items, function() {
  alert('this is ' + this);
});

var newItems = $.map(items, function(i) {
  return i + 1;
});
// newItems is [2,3,4,5]

You can also use the map function to remove an item from an array. For example:

var items = [0,1,2,3,4,5,6,7,8,9];

var itemsLessThanEqualFive = $.map(items, function(i) {
  // removes all items > 5
  if (i > 5) 
    return null;
  return i;
});
// itemsLessThanEqualFive = [0,1,2,3,4,5]

You'll also note that the this is not mapped in the map function. You will have to supply the first parameter in the callback (eg we used i above). Ironically, the callback arguments used in the each method are the reverse of the callback arguments in the map function so be careful.

map(arr, function(elem, index) {});
// versus 
each(arr, function(index, elem) {});

Answer 3

The each function iterates over an array, calling the supplied function once per element, and setting this to the active element. This:

function countdown() {
    alert(this + "..");
}

$([5, 4, 3, 2, 1]).each(countdown);

will alert 5.. then 4.. then 3.. then 2.. then 1..

Map on the other hand takes an array, and returns a new array with each element changed by the function. This:

function squared() {
    return this * this;
}

var s = $([5, 4, 3, 2, 1]).map(squared);

would result in s being [25, 16, 9, 4, 1].

Answer 4

1: The arguments to the callback function are reversed.

each()'s callback function takes the index first, and then the element

function (index, element)

map()'s callback has the same arguments, but reversed

function (element, index)

2: each() does something special with this

each() calls the function in such a way that this points to the current element. In most cases, you don't even need the two arguments in the callback function.

function shout() { alert(this + '!') }

result = $.each(['lions', 'tigers', 'bears'], shout)

// result == ['lions', 'tigers', 'bears']

3: map() does something special with the callback's return value

map() calls the function on each element, and stores the result in a new array, which it returns. You usually only need to use the first argument in the callback function.

function shout(el) { return el + '!' }

result = $.map(['lions', 'tigers', 'bears'], shout)

// result == ['lions!', 'tigers!', 'bears!']