I have a need to add or prepend elements at the beginning of an array.
For example, if my array looks like below:
[23, 45, 12, 67]
And the response from my AJAX call is 34
, I want the updated array to be like the following:
[34, 23, 45, 12, 67]
Currently I am planning to do it like this:
var newArray = [];
newArray.push(response);
for (var i = 0; i < theArray.length; i++) {
newArray.push(theArray[i]);
}
theArray = newArray;
delete newArray;
Is there a better way to do this? Does JavaScript have any built-in functionality that does this?
The complexity of my method is O(n)
and it would be really interesting to see better implementations.
push
statements followed by a call to reverse
, instead of calling unshift
all the time.
Use unshift
. It's like push
, except it adds elements to the beginning of the array instead of the end.
unshift/push - add an element to the beginning/end of an array
shift/pop - remove and return the first/last element of an array
A simple diagram...
unshift -> [array] <- push
shift <- [array] -> pop
and chart:
add remove start end
push X X
pop X X
unshift X X
shift X X
Check out the MDN Array documentation. Virtually every language that has the ability to push/pop elements from an array will also have the ability to unshift/shift (sometimes called push_front
/pop_front
) elements, you should never have to implement these yourself.
As pointed out in the comments, if you want to avoid mutating your original array, you can use concat
, which concatenates two or more arrays together. You can use this to functionally push a single element onto the front or back of an existing array; to do so, you need to turn the new element into a single element array:
const array = [3, 2, 1] const newFirstElement = 4 const newArray = [newFirstElement].concat(array) // [ 4, 3, 2, 1 ] console.log(newArray);
concat
can also append items. The arguments to concat
can be of any type; they are implicitly wrapped in a single-element array, if they are not already an array:
const array = [3, 2, 1] const newLastElement = 0 // Both of these lines are equivalent: const newArray1 = array.concat(newLastElement) // [ 3, 2, 1, 0 ] const newArray2 = array.concat([newLastElement]) // [ 3, 2, 1, 0 ] console.log(newArray1); console.log(newArray2);
https://i.stack.imgur.com/1pQk8.jpg
var a = [23, 45, 12, 67]; a.unshift(34); console.log(a); // [34, 23, 45, 12, 67]
[23, 45, 12, 67].unshift(34)
will not work. The Array must first be saved inside a variable, because unshift
itself returns a value.
With ES6, use the spread operator ...:
Demo
var arr = [23, 45, 12, 67]; arr = [34, ...arr]; // RESULT : [34,23, 45, 12, 67] console.log(arr)
concat
instead.
Another way to do that is through concat
:
var arr = [1, 2, 3, 4, 5, 6, 7]; console.log([0].concat(arr));
The difference between concat
and unshift
is that concat
returns a new array. The performance between them could be found here.
function fn_unshift() {
arr.unshift(0);
return arr;
}
function fn_concat_init() {
return [0].concat(arr)
}
Here is the test result:
https://i.stack.imgur.com/f5u3L.png
unshift
: 25,510 ±3.18% 99% slower concat
: 2,436,894 ±3.39% fastest
Quick Cheatsheet:
The terms shift/unshift and push/pop can be a bit confusing, at least to folks who may not be familiar with programming in C.
If you are not familiar with the lingo, here is a quick translation of alternate terms, which may be easier to remember:
* array_unshift() - (aka Prepend ;; InsertBefore ;; InsertAtBegin )
* array_shift() - (aka UnPrepend ;; RemoveBefore ;; RemoveFromBegin )
* array_push() - (aka Append ;; InsertAfter ;; InsertAtEnd )
* array_pop() - (aka UnAppend ;; RemoveAfter ;; RemoveFromEnd )
Using ES6 destructuring (avoiding mutation off the original array):
const newArr = [item, ...oldArr]
Without Mutating
Actually, all unshift
/push
and shift
/pop
mutate the source array.
The unshift
/push
add an item to the existed array from begin/end and shift
/pop
remove an item from the beginning/end of an array.
But there are few ways to add items to an array without a mutation. the result is a new array, to add to the end of array use below code:
const originArray = ['one', 'two', 'three'];
const newItem = 4;
const newArray = originArray.concat(newItem); // ES5
const newArray2 = [...originArray, newItem]; // ES6+
To add to begin of original array use below code:
const originArray = ['one', 'two', 'three'];
const newItem = 0;
const newArray = (originArray.slice().reverse().concat(newItem)).reverse(); // ES5
const newArray2 = [newItem, ...originArray]; // ES6+
With the above way, you add to the beginning/end of an array without a mutation.
slice
function at the end of originArray
to prevent it from mutability.
You have an array: var arr = [23, 45, 12, 67];
To add an item to the beginning, you want to use splice
:
var arr = [23, 45, 12, 67]; arr.splice(0, 0, 34) console.log(arr);
unshift
50% faster (but mostly more readable)
Cheatsheet to prepend new element(s) into the array
const list = [23, 45, 12, 67]; list.unshift(34); console.log(list); // [34, 23, 45, 12, 67];
2. Array#splice
const list = [23, 45, 12, 67]; list.splice(0, 0, 34); console.log(list); // [34, 23, 45, 12, 67];
const list = [23, 45, 12, 67]; const newList = [34, ...list]; console.log(newList); // [34, 23, 45, 12, 67];
4. Array#concat
const list = [23, 45, 12, 67]; const newList = [32].concat(list); console.log(newList); // [34, 23, 45, 12, 67];
Note: In each of these examples, you can prepend multiple items by providing more items to insert.
If you need to continuously insert an element at the beginning of an array, it is faster to use push
statements followed by a call to reverse
, instead of calling unshift
all the time.
Benchmark test: http://jsben.ch/kLIYf
unshift
across all major desktop browsers (at least on this Mac).
Using splice
we insert an element to an array at the begnning:
arrName.splice( 0, 0, 'newName1' );
If you want to push elements that are in an array at the beginning of your array, use <func>.apply(<this>, <Array of args>)
:
const arr = [1, 2]; arr.unshift.apply(arr, [3, 4]); console.log(arr); // [3, 4, 1, 2]
There are a couple of ways to achieve this:
Using shift (mutable)
const num = 1; const arr = [2, 3]; arr.unshift(num); console.log(arr);
Using ES6 destructuring (immutable)
const num = 1; const arr = [2, 3]; const newArr = [num, ...arr]; console.log(newArr);
Using Array.prototype.concat (immutable)
const num = 1; const arr = [2, 3]; const newArr = [num].concat(arr); console.log(newArr);
let arr = [5, 6]; // using unshift arr.unshift(4); console.log('arr : ', arr); // using spread operator arr = [3, ...arr]; console.log('arr : ', arr); // using concat arr = [2].concat(arr); console.log('arr : ', arr); // using splice arr.splice(0, 0, 1) console.log('arr : ', arr);
Success story sharing
concat
might be preferable as it returns the new array. Very useful for chaining.[thingToInsertToFront].concat(originalArray).reduce(fn).reverse().map(fn)
etc... If you useunshift
, you can't do that chaining because all you get back is the length.