Copy array items into another array

Copy array items into another array

I have a JavaScript array dataArray which I want to push into a new array newArray. Except I don’t want newArray[0] to be dataArray. I want to push in all the items into the new array:
var newArray = [];

newArray.pushValues(dataArray1);
newArray.pushValues(dataArray2);
// …

or even better:
var newArray = new Array (
dataArray1.values(),
dataArray2.values(),
// … where values() (or something equivalent) would push the individual values into the array, rather than the array itself
);

So now the new array contains all the values of the individual data arrays. Is there some shorthand like pushValues available so I don’t have to iterate over each individual dataArray, adding the items one by one?

Solutions/Answers:

Solution 1:

Use the concat function, like so:

var arrayA = [1, 2];
var arrayB = [3, 4];
var newArray = arrayA.concat(arrayB);

The value of newArray will be [1, 2, 3, 4] (arrayA and arrayB remain unchanged; concat creates and returns a new array for the result).

Solution 2:

Provided your arrays are not huge (see caveat below), you can use the push() method of the array to which you wish to append values. push() can take multiple parameters so you can use its apply() method to pass the array of values to be pushed as a list of function parameters. This has the advantage over using concat() of adding elements to the array in place rather than creating a new array.

However, it seems that for large arrays (of the order of 100,000 members or more), this trick can fail. For such arrays, using a loop is a better approach. See https://stackoverflow.com/a/17368101/96100 for details.

var newArray = [];
newArray.push.apply(newArray, dataArray1);
newArray.push.apply(newArray, dataArray2);

You might want to generalize this into a function:

function pushArray(arr, arr2) {
    arr.push.apply(arr, arr2);
}

… or add it to Array‘s prototype:

Array.prototype.pushArray = function(arr) {
    this.push.apply(this, arr);
};

var newArray = [];
newArray.pushArray(dataArray1);
newArray.pushArray(dataArray2);

… or emulate the original push() method by allowing multiple parameters using the fact that concat(), like push(), allows multiple parameters:

Array.prototype.pushArray = function() {
    this.push.apply(this, this.concat.apply([], arguments));
};

var newArray = [];
newArray.pushArray(dataArray1, dataArray2);

Here’s a loop-based version of the last example, suitable for large arrays and all major browsers, including IE <= 8:

Array.prototype.pushArray = function() {
    var toPush = this.concat.apply([], arguments);
    for (var i = 0, len = toPush.length; i < len; ++i) {
        this.push(toPush[i]);
    }
};

Solution 3:

I will add one more “future-proof” reply

In ECMAScript 6, you can use the Spread syntax:

let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
arr1.push(...arr2);

console.log(arr1)

Spread syntax is not yet included in all major browsers. For the current compatibility, see this (continuously updated) compatibility table.

You can, however, use spread syntax with Babel.js.

edit:

See Jack Giffin’s reply below for more comments on performance. It seems concat is still better and faster than spread operator.

Solution 4:

Found an elegant way from MDN

var vegetables = ['parsnip', 'potato'];
var moreVegs = ['celery', 'beetroot'];

// Merge the second array into the first one
// Equivalent to vegetables.push('celery', 'beetroot');
Array.prototype.push.apply(vegetables, moreVegs);

console.log(vegetables); // ['parsnip', 'potato', 'celery', 'beetroot']

Or you can use the spread operator feature of ES6:

let fruits = [ 'apple', 'banana'];
const moreFruits = [ 'orange', 'plum' ];

fruits.push(...moreFruits); // ["apple", "banana", "orange", "plum"]

Solution 5:

var a=new Array('a','b','c');
var b=new Array('d','e','f');
var d=new Array('x','y','z');
var c=a.concat(b,d)

Does that solve your problem ?

Solution 6:

The following seems simplest to me:

var newArray = dataArray1.slice();
newArray.push.apply(newArray, dataArray2);

As “push” takes a variable number of arguments, you can use the apply method of the push function to push all of the elements of another array. It constructs
a call to push using its first argument (“newArray” here) as “this” and the
elements of the array as the remaining arguments.

The slice in the first statement gets a copy of the first array, so you don’t modify it.

Update If you are using a version of javascript with slice available, you can simplify the push expression to:

newArray.push(...dataArray2)