What use does the JavaScript forEach method have (that map can't do)?

Question

The only difference I see in map and foreach is that map is returning an array and forEach is not. However, I don't even understand the last line of the forEach method "func.call(scope, this[i], i, this);". For example, isn't "this" and "scope" referring to same object and isn't this[i] and i referring to the current value in the loop?

I noticed on another post someone said "Use forEach when you want to do something on the basis of each element of the list. You might be adding things to the page, for example. Essentially, it's great for when you want "side effects". I don't know what is meant by side effects.

Array.prototype.map = function(fnc) {
    var a = new Array(this.length);
    for (var i = 0; i < this.length; i++) {
        a[i] = fnc(this[i]);
    }
    return a;
}

Array.prototype.forEach = function(func, scope) {
    scope = scope || this;
    for (var i = 0, l = this.length; i < l; i++) {
        func.call(scope, this[i], i, this);
    }
}

Finally, are there any real uses for these methods in JavaScript (since we aren't updating a database) other than to manipulate numbers like the following?

alert([1,2,3,4].map(function(x){ return x + 1})); // This is the only example I ever see of map in JavaScript.

Answer 1

The essential difference between map and forEach in your example is that forEach operates on the original array elements, whereas map explicitly returns a new array as a result.

With forEach you are taking some action with -- and optionally changing -- each element in the original array. The forEach method runs the function you provide for each element, but returns nothing (undefined). On the other hand, map walks through the array, applies a function to each element, and emits the result as a new array.

The "side effect" with forEach is that the original array is being changed. "No side effect" with map means that, in idiomatic usage, the original array elements are not changed; the new array is a one-to-one mapping of each element in the original array -- the mapping transform being your provided function.

The fact that there's no database involved does not mean that you won't have to operate on data structures, which, after all, is one of the essences of programming in any language. As for your last question, your array can contain not only numbers, but objects, strings, functions, etc.

Answer 2

The main difference between the two methods is conceptual and stylistic: You use forEach when you want to do something to or with each element of an array (doing "with" is what the post you cite meant by "side-effects", I think), whereas you use map when you want to copy and transform each element of an array (without changing the original).

Because both map and forEach call a function on each item in an array, and that function is user-defined, there is almost nothing you can do with one and not with the other. It's possible, though ugly, to use map to modify an array in-place and/or do something with array elements:

var a = [{ val: 1 }, { val: 2 }, { val: 3 }];
a.map(function(el) {
    el.val++; // modify element in-place
    alert(el.val); // do something with each element
});
// a now contains [{ val: 2 }, { val: 3 }, { val: 4 }]

but much cleaner and more obvious as to your intent to use forEach:

var a = [{ val: 1 }, { val: 2 }, { val: 3 }];
a.forEach(function(el) {
    el.val++;
    alert(el.val);
});

Especially if, as is usually the case in the real world, el is a usefully human-readable variable:

cats.forEach(function(cat) {
    cat.meow(); // nicer than cats[x].meow()
});

In the same way, you can easily use forEach to make a new array:

var a = [1,2,3],
    b = [];
a.forEach(function(el) {
    b.push(el+1);
});
// b is now [2,3,4], a is unchanged

but it's cleaner to use map:

var a = [1,2,3],
    b = a.map(function(el) {
        return el+1;
    });

Note as well that, because map makes a new array, it likely incurs at least some performance/memory hit when all you need is iteration, particularly for large arrays - see http://jsperf.com/map-foreach

As for why you'd want to use these functions, they're helpful any time you need to do array manipulation in JavaScript, which (even if we're just talking about JavaScript in a browser environment) is pretty often, almost any time you're accessing an array that you're not writing down by hand in your code. You might be dealing with an array of DOM elements on the page, or data pulled from an Ajax request, or data entered in a form by the user. One common example I run into is pulling data from an external API, where you might want to use map to transform the data into the format you want and then use forEach to iterate over your new array in order to display it to your user.

Answer 3

The voted answer (from Ken Redler) is misleading.

A side effect in computer science means that a property of a function/method alters a global state [Wikipedia]. In some narrow sense, this may also include reading from a global state, rather than from arguments. In imperative or OO programming, side effects appear most of the time. And you are probably making use of it without realizing.

The significant difference between forEach and map is that map allocates memory and stores the returning value, while forEach throws it away. See the ECMA specification for more information.

As for the reason why people say forEach is used when you want a side effect is that the return value of forEach is always undefined. If it has no side effect (does not change global state), then the function is just wasting CPU time. An optimizing compiler will eliminate this code block and replace the it with the final value (undefined).

By the way, it should be noted that JavaScript has no restriction on side effects. You can still modify the original array inside map.

var a = [1,2,3]; //original
var b = a.map( function(x,i){a[i] = 2*x; return x+1} );
console.log("modified=%j\nnew array=%j",a,b);
// output:
// modified=[2,4,6]
// new array=[2,3,4]

Answer 4

This is a beautiful question with an unexpected answer.

The following is based on the official description of Array.prototype.map().

There is nothing that forEach() can do that map() cannot. That is, map() is a strict super-set of forEach().

Although map() is usually used to create a new array, it may also be used to change the current array. The following example illustrates this:

var a = [0, 1, 2, 3, 4], mapped = null;
mapped = a.map(function (x) { a[x] = x*x*x; return x*x; });
console.log(mapped); // logs [0, 1, 4, 9, 16]  As expected, these are squares.
console.log(a); // logs [0, 1, 8, 27, 64] These are cubes of the original array!!

In the above example, a was conveniently set such that a[i] === i for i < a.length. Even so, it demonstrates the power of map(), and in particular its ability to change the array on which it is called.

Note1:
The official description implies that map() may even change length the array on which it is called! However, I cannot see (a good) reason to do this.

Note 2:
While map() map is a super-set of forEach(), forEach() should still be used where one desires the change a given array. This makes your intentions clear.

Answer 5

You can use map as though it were forEach.

It will do more than it has to, however.

scope can be an arbitrary object; it's by no means necessarily this.

As for whether there are real uses for map and forEach, as well to ask if there are real uses for for or while loops.

Answer 6

While all the previous questions are correct, I would definitely make a different distinction. The use of map and forEach can imply intent.

I like to use map when I am simply transforming the existing data in some way (but want to make sure the original data is unchanged).

I like to use forEach when I am modifying the collection in place.

For instance,

var b = [{ val: 1 }, { val: 2 }, { val: 3 }];
var c = b.map(function(el) {
    return { val: el.val + 1 }; // modify element in-place
});
console.log(b);
//  [{ val: 1 }, { val: 2 }, { val: 3 }]
console.log(c);
//  [{ val: 3 }, { val: 4 }, { val: 5 }]

My rule of thumb being making sure when you map you are always creating some new object/value to return for each element of the source list and returning it rather than just performing some operation on each element.

Unless you have any real need to modify the existing list, it doesn't really make sense to modify it in place and fits better into functional/immutable programming styles.

Answer 7

TL;DR answer --

map always returns another array.

forEach does not. It is up to you to decide what it does. Return an array if you want or do something else if you don't.

Flexibility is desirable is certain situations. If it isn't for what you are dealing with then use map.

Answer 8

Others have already posted about your main question regarding the difference between the functions. But for...

are there any real uses for these methods in JavaScript (since we aren't updating a database) other than to manipulate numbers like this:

...it's funny you should ask. Just today I wrote a piece of code that assigns a number of values from a regular expression to multiple variables using map for transformation.

It was used to convert a very complicated text-based structure into visualizable data ... but for simplicity's sake, I shall offer an example using date strings, because those are probably more familiar for everyone (though, if my problem had actually been with dates, instead of map I would've used Date-object, which would've done the job splendidly on its own).

const DATE_REGEXP = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})\.(\d{3})Z$/;
const TEST_STRING = '2016-01-04T03:20:00.000Z';

var [
    iYear,
    iMonth,
    iDay,
    iHour,
    iMinute,
    iSecond,
    iMillisecond
    ] = DATE_REGEXP
        // We take our regular expression and...
        .exec(TEST_STRING)
        // ...execute it against our string (resulting in an array of matches)...
        .slice(1)
        // ...drop the 0th element from those (which is the "full string match")...
        .map(value => parseInt(value, 10));
        // ...and map the rest of the values to integers...

// ...which we now have as individual variables at our perusal
console.debug('RESULT =>', iYear, iMonth, iDay, iHour, iMinute, iSecond, iMillisecond);

So ... while this was just an example - and only did a very basic transformation for the data (just for sake of example) ... having done this without map would've been a much more tedious task.

Granted, it is written in a version of JavaScript that I don't think too many browsers support yet (at least fully), but - we're getting there. If I needed to run it in browser, I believe it would transpile nicely.