There doesn't seem to be a way to extend an existing JavaScript array with another array, i.e. to emulate Python's extend method.
I want to achieve the following:
>>> a = [1, 2]
[1, 2]
>>> b = [3, 4, 5]
[3, 4, 5]
>>> SOMETHING HERE
>>> a
[1, 2, 3, 4, 5]
I know there's a a.concat(b) method, but it creates a new array instead of simply extending the first one. I'd like an algorithm that works efficiently when a is significantly larger than b (i.e. one that does not copy a).
Note: This is not a duplicate of How to append something to an array? -- the goal here is to add the whole contents of one array to the other, and to do it "in place", i.e. without copying all elements of the extended array.
The .push method can take multiple arguments. You can use the spread operator to pass all the elements of the second array as arguments to .push:
>>> a.push(...b)
If your browser does not support ECMAScript 6, you can use .apply instead:
>>> a.push.apply(a, b)
Or perhaps, if you think it's clearer:
>>> Array.prototype.push.apply(a,b)
Please note that all these solutions will fail with a stack overflow error if array b is too long (trouble starts at about 100,000 elements, depending on the browser).
If you cannot guarantee that b is short enough, you should use a standard loop-based technique described in the other answer.
Update 2018: A better answer is a newer one of mine: a.push(...b). Don't upvote this one anymore, as it never really answered the question, but it was a 2015 hack around first-hit-on-Google :)
For those that simply searched for "JavaScript array extend" and got here, you can very well use Array.concat.
var a = [1, 2, 3];
a = a.concat([5, 4, 3]);
Concat will return a copy the new array, as thread starter didn't want. But you might not care (certainly for most kind of uses this will be fine).
There's also some nice ECMAScript 6 sugar for this in the form of the spread operator:
const a = [1, 2, 3];
const b = [...a, 5, 4, 3];
(It also copies.)
You should use a loop-based technique. Other answers on this page that are based on using .apply can fail for large arrays.
A fairly terse loop-based implementation is:
Array.prototype.extend = function (other_array) {
/* You should include a test to check whether other_array really is an array */
other_array.forEach(function(v) {this.push(v)}, this);
}
You can then do the following:
var a = [1,2,3];
var b = [5,4,3];
a.extend(b);
DzinX's answer (using push.apply) and other .apply based methods fail when the array that we are appending is large (tests show that for me large is > 150,000 entries approx in Chrome, and > 500,000 entries in Firefox). You can see this error occurring in this jsperf.
An error occurs because the call stack size is exceeded when 'Function.prototype.apply' is called with a large array as the second argument. (MDN has a note on the dangers of exceeding call stack size using Function.prototype.apply - see the section titled "apply and built-in functions".)
For a speed comparison with other answers on this page, check out this jsperf (thanks to EaterOfCode). The loop-based implementation is similar in speed to using Array.push.apply, but tends to be a little slower than Array.slice.apply.
Interestingly, if the array you are appending is sparse, the forEach based method above can take advantage of the sparsity and outperform the .apply based methods; check out this jsperf if you want to test this for yourself.
By the way, do not be tempted (as I was!) to further shorten the forEach implementation to:
Array.prototype.extend = function (array) {
array.forEach(this.push, this);
}
because this produces garbage results! Why? Because Array.prototype.forEach provides three arguments to the function it calls - these are: (element_value, element_index, source_array). All of these will be pushed onto your first array for every iteration of forEach if you use "forEach(this.push, this)"!
I feel the most elegant these days is:
arr1.push(...arr2);
The MDN article on the spread operator mentions this nice sugary way in ES2015 (ES6):
A better push
Example: push is often used to push an array to the end of an existing
array. In ES5 this is often done as:
var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
// Append all items from arr2 onto arr1
Array.prototype.push.apply(arr1, arr2);
In ES6 with spread this becomes:
var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
arr1.push(...arr2);
Do note that arr2 can't be huge (keep it under about 100 000 items), because the call stack overflows, as per jcdude's answer.
Overview
a.push(...b) - limited, fast, modern syntax
a.push.apply(a, b) - limited, fast
a = a.concat(b) unlimited, slow if a is large
for (let i in b) { a.push(b[i]); } - unlimited, slow if b is large
Each snippet modifies a to be extended with b.
The "limited" snippets pass each array element as an argument, and the maximum number of arguments you can pass to a function is limited. From that link, it seems that a.push(...b) is reliable until there are about 32k elements in b (the size of a does not matter).
Relevant MDN documentation: spread syntax, .apply(), .concat(), .push()
Speed considerations
Every method is fast if both a and b are small, so in most web applications you'll want to use push(...b) and be done with it.
If you're handling more than a few thousand elements, what you want to do depends on the situation:
you're adding a few elements to a large array
→ push(...b) is very fast
you're adding many elements to a large array
→ concat is slightly faster than a loop
you're adding many elements to a small array
→ concat is much faster than a loop
you're usually adding only a few elements to any size array
→ loops are about as fast as the limited methods for small additions, but will never throw an exception even if it is not the most performant when you add many elements
you're writing a wrapper function to always get the maximum performance
→ you'll need to check the lengths of the inputs dynamically and choose the right method, perhaps calling push(...b_part) (with slices of the big b) in a loop.
This surprised me: I thought a=a.concat(b) would be able to do a nice memcpy of b onto a without bothering to do individual extend operations as a.push(...b) would have to do, thus always being the fastest. Instead, a.push(...b) is much, much faster especially when a is large.
The speed of different methods was measured in Firefox 88 on Linux using:
a = [];
for (let i = 0; i < Asize; i++){
a.push(i);
}
b = [];
for (let i = 0; i < Bsize; i++){
b.push({something: i});
}
t=performance.now();
// Code to test
console.log(performance.now() - t);
Parameters and results:
ms | Asize | Bsize | code
----+-------+-------+------------------------------
~0 | any | any | a.push(...b)
~0 | any | any | a.push.apply(a, b)
480 | 10M | 50 | a = a.concat(b)
0 | 10M | 50 | for (let i in b) a.push(b[i])
506 | 10M | 500k | a = a.concat(b)
882 | 10M | 500k | for (let i in b) a.push(b[i])
11 | 10 | 500k | a = a.concat(b)
851 | 10 | 500k | for (let i in b) a.push(b[i])
Note that a Bsize of 500 000 is the largest value accepted by all methods on my system, that's why it is smaller than Asize.
All tests were run multiple times to see if the results are outliers or representative. The fast methods are almost immeasurable in just one run using performance.now(), of course, but since the slow methods are so obvious and the two fast methods both work on the same principle, we needn't bother repeating it a bunch of times to split hairs.
The concat method is always slow if either array is large, but the loop is only slow if it has to do a lot of function calls and doesn't care how large a is. A loop is thus similar to push(...b) or push.apply for small bs but without breaking if it does get large; however, when you approach the limit, concat is a bit faster again.
First a few words about apply() in JavaScript to help understand why we use it:
The apply() method calls a function with a given this value, and
arguments provided as an array.
Push expects a list of items to add to the array. The apply() method, however, takes the expected arguments for the function call as an array. This allows us to easily push the elements of one array into another array with the builtin push() method.
Imagine you have these arrays:
var a = [1, 2, 3, 4];
var b = [5, 6, 7];
and simply do this:
Array.prototype.push.apply(a, b);
The result will be:
a = [1, 2, 3, 4, 5, 6, 7];
The same thing can be done in ES6 using the spread operator ("...") like this:
a.push(...b); //a = [1, 2, 3, 4, 5, 6, 7];
Shorter and better but not fully supported in all browsers at the moment.
Also if you want to move everything from array b to a, emptying b in the process, you can do this:
while(b.length) {
a.push(b.shift());
}
and the result will be as follows:
a = [1, 2, 3, 4, 5, 6, 7];
b = [];
If you want to use jQuery, there is $.merge()
Example:
a = [1, 2];
b = [3, 4, 5];
$.merge(a,b);
Result: a = [1, 2, 3, 4, 5]
I like the a.push.apply(a, b) method described above, and if you want you can always create a library function like this:
Array.prototype.append = function(array)
{
this.push.apply(this, array)
}
and use it like this
a = [1,2]
b = [3,4]
a.append(b)
It is possible to do it using splice():
b.unshift(b.length)
b.unshift(a.length)
Array.prototype.splice.apply(a,b)
b.shift() // Restore b
b.shift() //
But despite being uglier it is not faster than push.apply, at least not in Firefox 3.0.
as the top voted answer says, a.push(...b) is probably the correct answer taking into account the size limit issue.
On the other hand, some of the answers on performance seem out of date.
These numbers below are for 2022-05-20
from here
At appears that push is fastest across the board in 2022. That may change in the future.
Answers ignoring the question (generating a new array) are missing the point. Lots of code might need/want to modify an array in place given there can be other references to the same array
let a = [1, 2, 3];
let b = [4, 5, 6];
let c = a;
a = a.concat(b); // a and c are no longer referencing the same array
Those other references could be deep in some object, something that was captured in a closure, etc...
As a probably bad design but as an illustration, imagine you had
const carts = [
{ userId: 123, cart: [item1, item2], },
{ userId: 456, cart: [item1, item2, item3], },
];
and a function
function getCartForUser(userId) {
return customers.find(c => c.userId === userId);
}
Then you want to add items to the cart
const cart = getCartForUser(userId);
if (cart) {
cart.concat(newItems); // FAIL 😢
cart.push(...newItems); // Success! 🤩
}
As an aside, the answers suggesting modifying Array.prototype are arguably bad adivce. Changing the native prototypes is bascially a landmine in your code. Another implementation maybe be different than yours and so it will break your code or you'll break their code expecting the other behavior. This includes if/when a native implmentation gets added that clashes with yours. You might say "I know what I'm using so no issue" and that might be true at the moment and you're a single dev but add a second dev and they can't read your mind. And, you are that second dev in a few years when you've forgotten and then graft some other library (analytics?, logging?, ...) on to your page and forget the landmind you left in the code.
This is not just theory. There are countless stories on the net of people running into these landmines.
Arguably there are just a few safe uses for modifying a native object's prototype. One is to polyfill an existing and specified implementation in an old browser. In that case, the spec is defined, the spec is implemented is shipping in new browsers, you just want to get the same behavior in old browsers. That's pretty safe. Pre-patching (spec in progress but not shipping) is arguably not safe. Specs change before shipping.
This solution works for me (using the spread operator of ECMAScript 6):
let array = ['my', 'solution', 'works'];
let newArray = [];
let newArray2 = [];
newArray.push(...array); // Adding to same array
newArray2.push([...array]); // Adding as child/leaf/sub-array
console.log(newArray);
console.log(newArray2);
I'm adding this answer, because despite the question stating clearly without creating a new array, pretty much every answer just ignores it.
Modern JavaScript works well with arrays and alike as iterable objects. This makes it possible to implement a version of concat that builds upon that, and spans the array data across its parameters logically.
The example below makes use of iter-ops library that features such logic:
import {pipe, concat} from 'iter-ops';
const i = pipe(
originalArray,
concat(array2, array3, array4, ...)
); //=> Iterable
for(const a of i) {
console.log(a); // iterate over values from all arrays
}
Above, no new array is created. Operator concat will iterate through the original array, then will automatically continue into array2, then array3, and so on, in the specified order.
This is the most efficient way of joining arrays in terms of memory usage.
And if, at the end, you decide to convert it into an actual physical array, you can do so via the spread operator or Array.from:
const fullArray1 = [...i]; // pulls all values from iterable, into a new array
const fullArray2 = Array.from(i); // does the same
Combining the answers...
Array.prototype.extend = function(array) {
if (array.length < 150000) {
this.push.apply(this, array)
} else {
for (var i = 0, len = array.length; i < len; ++i) {
this.push(array[i]);
};
}
}
You can create a polyfill for extend as I have below. It will add to the array; in-place and return itself, so that you can chain other methods.
if (Array.prototype.extend === undefined) {
Array.prototype.extend = function(other) {
this.push.apply(this, arguments.length > 1 ? arguments : other);
return this;
};
}
function print() {
document.body.innerHTML += [].map.call(arguments, function(item) {
return typeof item === 'object' ? JSON.stringify(item) : item;
}).join(' ') + '\n';
}
document.body.innerHTML = '';
var a = [1, 2, 3];
var b = [4, 5, 6];
print('Concat');
print('(1)', a.concat(b));
print('(2)', a.concat(b));
print('(3)', a.concat(4, 5, 6));
print('\nExtend');
print('(1)', a.extend(b));
print('(2)', a.extend(b));
print('(3)', a.extend(4, 5, 6));
body {
font-family: monospace;
white-space: pre;
}
Another solution to merge more than two arrays
var a = [1, 2],
b = [3, 4, 5],
c = [6, 7];
// Merge the contents of multiple arrays together into the first array
var mergeArrays = function() {
var i, len = arguments.length;
if (len > 1) {
for (i = 1; i < len; i++) {
arguments[0].push.apply(arguments[0], arguments[i]);
}
}
};
Then call and print as:
mergeArrays(a, b, c);
console.log(a)
Output will be: Array [1, 2, 3, 4, 5, 6, 7]
The answer is super simple.
>>> a = [1, 2]
[1, 2]
>>> b = [3, 4, 5]
[3, 4, 5]
>>> SOMETHING HERE
(The following code will combine the two arrays.)
a = a.concat(b);
>>> a
[1, 2, 3, 4, 5]
Concat acts very similarly to JavaScript string concatenation. It will return a combination of the parameter you put into the concat function on the end of the array you call the function on. The crux is that you have to assign the returned value to a variable or it gets lost. So for example
a.concat(b); <--- This does absolutely nothing since it is just returning the combined arrays, but it doesn't do anything with it.
Another option, if you have lodash installed:
import { merge } from 'lodash';
var arr1 = merge(arr1, arr2);
Use Array.extend instead of Array.push for > 150,000 records.
if (!Array.prototype.extend) {
Array.prototype.extend = function(arr) {
if (!Array.isArray(arr)) {
return this;
}
for (let record of arr) {
this.push(record);
}
return this;
};
}
You can do that by simply adding new elements to the array with the help of the push() method.
let colors = ["Red", "Blue", "Orange"];
console.log('Array before push: ' + colors);
// append new value to the array
colors.push("Green");
console.log('Array after push : ' + colors);
Another method is used for appending an element to the beginning of an array is the unshift() function, which adds and returns the new length. It accepts multiple arguments, attaches the indexes of existing elements, and finally returns the new length of an array:
let colors = ["Red", "Blue", "Orange"];
console.log('Array before unshift: ' + colors);
// append new value to the array
colors.unshift("Black", "Green");
console.log('Array after unshift : ' + colors);
There are other methods too. You can check them out here.
Super simple, does not count on spread operators or apply, if that's an issue.
b.map(x => a.push(x));
After running some performance tests on this, it's terribly slow, but answers the question in regards to not creating a new array. Concat is significantly faster, even jQuery's $.merge() whoops it.
https://jsperf.com/merge-arrays19b/1
Would appreciate some feedback. I read the MDN, but I still don't get it. Thank you
function frankenSplice(arr1, arr2, n) {
return arr2.splice(n,0,arr1);
}
console.log(frankenSplice([1, 2, 3], [4, 5, 6], 1));
As correctly mentioned above, splice is usually used for adding single elements. Nothing, however, prevents it from working with arrays and adding those as elements to the other arrays. The reason for returning an empty array is that it returns array of deleted elements, not new array.
If you want to return updated array, do
function frankenSplice(arr1, arr2, n) {
arr2.splice(n,0,arr1);
return arr2;
}
splice is used for concatenating singular elements to an array.
If you want to combine arrays, first splice the element to one array:
var arr1 = [1,2,3,4]; // elements could be anything, I am just using integers here.
arr1.splice(n,0,element);
Then use concat if you want to combine two arrays.
var arr1 = [1,2,3,4];
var arr2 = [5,6,7,8];
var result = arr1.concat(arr2);
The question you are asking might not be the question you want answered, but from the MDN:
Return value An array containing the deleted elements. If only one
element is removed, an array of one element is returned. If no
elements are removed, an empty array is returned.
You are expecting your function to return the spliced array, for which you need:
function frankenSplice(arr1, arr2, n) {
arr2.splice(n,0,arr1);
return arr2;
}
However, I suspect that #jshamble is right and you are not actually looking to use .splice().
Splice doesnt 'equal' the result, it directly writes it back to the variable. So instead of returning arr2.splice(n, 0, arr1); you have to write
arr2.splice(n, 0, arr1);
return arr2
In JavaScript, if you set an array to be of size 5 ( var foo = new Array(5); ), is this just an initial size? Can you expand the number of elements after it is created. Is it possible to do something like this as well - arr = new Array() and then just assign elements one by one? Thanks in advance :-)
Yes it is just an initial size, and it is not required. If you don't use a single number, you can immediately populate.
It is also more common to use the simpler [] syntax.
var arr = ['something', 34, 'hello'];
You can set (or replace) a specific index by using brackets:
arr[0] = "I'm here replacing whatever your first item was";
You can add to the end by using push:
arr.push('add me');
There may be faster performance in some browsers if you do it like this instead:
arr[arr.length] = 'add me';
You can add something at any index.
You can remove an item completely using splice:
arr.splice(0, 1); // remove first item in the array (start at index 0, with 1 being removed)
When you give a new array an explicit size in javascript (using new Array(5)), you populate each index with value of undefined. It is generally considered better practice to instantiate using the array literal [] expression:
var arr = [];
Then, you can push new elements using push()
var arr = [];
arr.push('first value'):
arr.push('second value'):
Check out the MDC Array documentation for more info: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array
Theres a few ways to declare and put values in an array.
First like what you want to do,
var myarr = new Array();
myarr[0] = 'element1';
myarr[1] = 'element2';
myarr[2] = 'element3';
Second way is to define them
var myarr =new Array("element1","element2","element3");
and third is similar to the second
var myarr =["element1","element2","element3"];
You can also check out https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array for a little more information about using the arrays as well. You could use push and pop if you wanted to as well.
If you use jquery or mootools they also have built-in functions to perform on arrays,
http://api.jquery.com/jQuery.each/ for instance.
Have a look at http://www.w3schools.com/js/js_obj_array.asp
var myCars=new Array(); // regular array (add an optional integer
myCars[0]="Saab"; // argument to control array's size)
myCars[1]="Volvo";
myCars[2]="BMW";
Check the documentation for Array, but the simple answer to your question is yes.
var arr5 = new Array(1, 2, 3, 4, 5); // an array with initial 5 elements
var arr = new Array(); // an array without initial
You can also use array literals:
var arr5 = [1, 2, 3, 4, 5];
var arr = [];
Arrays are dynamic in JavaScript. You don't have to initialize them with a certain length. In fact you should use the literal notation [] because of the Array constructor's ambiguity:
If you pass only one parameter to Array, it will set the array length to this parameter. If you pass more than one parameter, these elements are added to the array.
How is the size of the array determined?
The size of an array is the highest index + 1. This can be quite confusing. Consider this:
var arr = [];
arr[41] = 'The answer?';
console.log(arr); // [undefined, undefined, ..., 'The answer?']
console.log(arr.length) // 42
You can even set the length yourself by assigning a number to .length:
arr.length = 99;
If you now add a new element using arr.push(), it will get the index 100 and the length will increase. Whenever you add an element to the array via an index, it is tested whether arr.length is smaller than the index and updated accordingly. But it does not get smaller.
So in fact what var arr = new Array(5) is doing is setting the length of the array to 5. Nothing else.
For more information about creating and populating arrays, I suggest to read about it in the MDC JavaScript Guide.
Is there a built in way to append one list into another like so:
var a = [1,2,3];
a.append([4,5]);
// now a is [1,2,3,4,5];
concat() does something similar but returns the result. I want something that modifies the existing list like push()
push will work, but you also need to use apply.
var a = [1,2,3];
a.push.apply(a, [4,5])
If you are using ES6 you can use the spread operator
eg :-
listOne = [1,2,3]
listTwo = [4,5,6]
listTwo.push(...listOne)
var list = [1, 2];
People already showed you how to do this with:
push: list.push.apply(list, [3, 4]);
concat: list = list.concat([4, 5]);
But I want to tell about ES6 spread operator: list.push(...[3, 4]);.
Keep in mind that currently not many browsers support it (you can use transpilers).
How about this:
var a = [1, 2, 3];
a = a.concat([4, 5]);
// a is now [1, 2, 3, 4, 5]
Because I just wrote a web application where a lot of arrays have to be concatenated and where performance is critical, I tested which method is fastest in this jsperf. The results are quite interesting.
In my test, I add 10 elements to a list or array of 10,000 elements.
Here are the test cases, from fastest to slowest. Results are measured in Chrome 62, but Firefox 47 performs similarly:
LinkedList.prototype.concat: 90,313,485 ops/sec
list.concat(concatList);
// This function has to change only 1-2 refences
Array.prototype.push in a for loop: 3,794,962 ops/sec
for (var i = 0, len = concatArr.length; i < len; i++) {
array.push(concatArr[i]);
}
// Probably fastest in real life
Array.prototype.push.apply: 2,193,469 ops/sec
array.push.apply(array, concatArr);
Array.prototype.concat: 22,701 ops/sec
array = array.concat(concatArr);
Unfortunately, the LinkedList version doesn't work if you want to concat an array/list to multiple LinkedLists. It also has no benefit if an array has to be copied to a LinkedList before every concat operation. So, for me, the winner is the for loop.
One reason to not use Array.prototype.push.apply is that it fails if the concatenated array is too big. According to this answer, the concatenated array can't have more than 500,000 elements in Firefox and 150,000 elements in Chrome.
I excluded the spread operator because it is not supported by all browsers. In Chrome, it's about as fast as Array.prototype.push.apply, in Firefox it is a bit slower.