Loop through an array

03/21/2020 04:00:02

In Java you can use a for loop to traverse objects in an array as follows:

String[] myStringArray = {"Hello", "World"};
for (String s : myStringArray)
{
    // Do something
}

Can you do the same in JavaScript?

Answer #1 (3844 Votes)

06/10/2010 05:07:12

You have several options:

1. Sequential for loop:

var myStringArray = ["Hello","World"];
var arrayLength = myStringArray.length;
for (var i = 0; i < arrayLength; i++) {
    console.log(myStringArray[i]);
    //Do something
}

Pros

  • Works on every environment
  • You can use break and continue flow control statements

Cons

  • Too verbose
  • Imperative
  • Easy to have off-by-one errors (sometimes also called a fence post error)

2. Array.prototype.forEach

The ES5 specification introduced a lot of beneficial array methods, one of them, the Array.prototype.forEach and it gives us a concise way to iterate over an array:

const array = ["one", "two", "three"]
array.forEach(function (item, index) {
  console.log(item, index);
});

Being almost ten years as the time of writing that the ES5 specification was released (Dec. 2009), it has been implemented by nearly all modern engines in the desktop, server, and mobile environments, so it's safe to use them.

And with the ES6 arrow function syntax, it's even more succinct:

array.forEach(item => console.log(item));

Arrow functions are also widely implemented unless you plan to support ancient platforms (e.g., IE11); you are also safe to go.

Pros

  • Very short and succinct.
  • Declarative

Cons

  • Cannot use break / continue

Normally, you can replace the need to break out of imperative loops by filtering the array elements before iterating them, for example:

array.filter(item => item.condition < 10)
     .forEach(item => console.log(item))

Keep in mind if you are iterating an array to build another array from it, you should use map, I've seen this anti-pattern so many times.

Anti-pattern:

const numbers = [1,2,3,4,5], doubled = [];

numbers.forEach((n, i) => { doubled[i] = n * 2 });

Proper use case of map:

const numbers = [1,2,3,4,5];
const doubled = numbers.map(n => n * 2);

console.log(doubled);

Also, if you are trying to reduce the array to a value, for example, you want to sum an array of numbers, you should use the reduce method.

Anti-pattern:

const numbers = [1,2,3,4,5];
const sum = 0;
numbers.forEach(num => { sum += num });

Proper use of reduce:

const numbers = [1,2,3,4,5];
const sum = numbers.reduce((total, n) => total + n, 0);

console.log(sum);

3. ES6 for-of statement

The ES6 standard introduces the concept of iterable objects and defines a new construct for traversing data, the for...of statement.

This statement works for any kind of iterable object and also for generators (any object that has a [Symbol.iterator] property).

Array objects are by definition built-in iterables in ES6, so you can use this statement on them:

let colors = ['red', 'green', 'blue'];
for (const color of colors){
    console.log(color);
}

Pros

  • It can iterate over a large variety of objects.
  • Can use normal flow control statements (break / continue).
  • Useful to iterate serially asynchronous values.

Cons

Do not use for...in

@zipcodeman suggests the use of the for...in statement, but for iterating arrays for-in should be avoided, that statement is meant to enumerate object properties.

It shouldn't be used for array-like objects because:

  • The order of iteration is not guaranteed; the array indexes may not be visited in numeric order.
  • Inherited properties are also enumerated.

The second point is that it can give you a lot of problems, for example, if you extend the Array.prototype object to include a method there, that property will also be enumerated.

For example:

Array.prototype.foo = "foo!";
var array = ['a', 'b', 'c'];

for (var i in array) {
    console.log(array[i]);
}

The above code will console log "a", "b", "c", and "foo!".

That be particularly a problem if you use some library that relies heavily on native prototypes augmentation (such as MooTools, for example).

The for-in statement as I said before is there to enumerate object properties, for example:

var obj = {
    "a": 1,
    "b": 2,
    "c": 3
};

for (var prop in obj) {
    if (obj.hasOwnProperty(prop)) { 
        // or if (Object.prototype.hasOwnProperty.call(obj,prop)) for safety...
        console.log("prop: " + prop + " value: " + obj[prop])
    }
}

In the above example, the hasOwnProperty method allows you to enumerate only own properties, that's it, only the properties that the object physically has, no inherited properties.

I would recommend you to read the following article:

3844

Answer #2 (1109 Votes)

04/16/2012 07:03:36

Yes, assuming your implementation includes the for...of feature introduced in ECMAScript 2015 (the "Harmony" release)... which is a pretty safe assumption these days.

It works like this:

// REQUIRES ECMASCRIPT 2015+
var s, myStringArray = ["Hello", "World"];
for (s of myStringArray) {
  // ... do something with s ...
}

Or better yet, since ECMAScript 2015 also provides block-scoped variables:

// REQUIRES ECMASCRIPT 2015+
const myStringArray = ["Hello", "World"];
for (const s of myStringArray) {
  // ... do something with s ...
}
// s is no longer defined here

(The variable s is different on each iteration, but can still be declared const inside the loop body as long as it isn't modified there.)

A note on sparse arrays: an array in JavaScript may not actually store as many items as reported by its length; that reported number is simply one greater than the highest index at which a value is stored. If the array holds fewer elements than indicated by its length, its said to be sparse. For example, it's perfectly legitimate to have an array with items only at indexes 3, 12, and 247; the length of such an array is reported as 248, though it is only actually storing 3 values. If you try to access an item at any other index, the array will appear to have the undefined value there. So when you want to "loop through" an array, you have a question to answer: do you want to loop over the full range indicated by its length and process undefineds for any missing elements, or do you only want to process the elements actually present? There are plenty of applications for both approaches; it just depends on what you're using the array for.

If you iterate over an array with for..of, the body of the loop is executed length times, and the loop control variable is set to undefined for any items not actually present in the array. Depending on the details of your "do something with" code, that behavior may be what you want, but if not, you should use a different approach.

Of course, some developers have no choice but to use a different approach anyway, because for whatever reason they're targeting a version of JavaScript that doesn't yet support for...of.

As long as your JavaScript implementation is compliant with the previous edition of the ECMAScript specification (which rules out, for example, versions of Internet Explorer before 9), then you can use the Array#forEach iterator method instead of a loop. In that case, you pass a function to be called on each item in the array:

var myStringArray = [ "Hello", "World" ];
myStringArray.forEach( function(s) { 
     // ... do something with s ...
} );

Unlike for...of, .forEach only calls the function for elements that are actually present in the array. If passed our hypothetical array with three elements and a length of 248, it will only call the function three times, not 248 times. It also distinguishes between missing elements and elements that are actually set to undefined; for the latter, it will still call the function, passing undefined as the argument. If this is how you want to handle sparse arrays, .forEach may be the way to go even if your interpreter supports for...of.

The final option, which works in all versions of JavaScript, is an explicit counting loop. You simply count from 0 up to one less than the length and use the counter as an index. The basic loop looks like this:

var i, s, myStringArray = [ "Hello", "World" ], len = myStringArray.length;
for (i=0; i<len; ++i) {
  s = myStringArray[i];
  // ... do something with s ...
}

One advantage of this approach is that you can choose how to handle sparse arrays; the above code will run the body of the loop the full length times, with s set to undefined for any missing elements, just like for..of. If you instead want to handle only the actually-present elements of a sparse array, like .forEach, you can add a simple in test on the index:

var i, s, myStringArray = [ "Hello", "World" ], len = myStringArray.length;
for (i=0; i<len; ++i) {
  if (i in myStringArray) {
    s = myStringArray[i];
    // ... do something with s ...
  }
}

Assigning the length value to the local variable (as opposed to including the full myStringArray.length expression in the loop condition) can make a significant difference in performance since it skips a property lookup each time through; using Rhino on my machine, the speedup is 43%.

You may see the length caching done in the loop initialization clause, like this:

var i, len, myStringArray = [ "Hello", "World" ];
for (len = myStringArray.length, i=0; i<len; ++i) {

The explicit counting loop also means you have access to the index of each value, should you want it. The index is also passed as an extra parameter to the function you pass to forEach, so you can access it that way as well:

myStringArray.forEach( function(s, i) {
   // ... do something with s and i ...
});

for...of doesn't give you the index associated with each object, but as long as the object you're iterating over is actually an Array (for..of works for other iterable types which may not have this method), you can use the Array#entries method to change it to an array of [index, item] pairs, and then iterate over that:

for (const [i, s] of myStringArray.entries()) {
  // ... do something with s and i ...
}

The for...in syntax mentioned by others is for looping over an object's properties; since an Array in JavaScript is just an object with numeric property names (and an automatically-updated length property), you can theoretically loop over an Array with it. But the problem is that it doesn't restrict itself to the numeric property values (remember that even methods are actually just properties whose value is a closure), nor is it guaranteed to iterate over those in numeric order. Therefore, the for...in syntax should not be used for looping through Arrays.

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