Copy array by value

Copy array by value

When copying an array in JavaScript to another array:
var arr1 = [‘a’,’b’,’c’];
var arr2 = arr1;
arr2.push(‘d’); //Now, arr1 = [‘a’,’b’,’c’,’d’]

I realized that arr2 refers to the same array as arr1, rather than a new, independent array. How can I copy the array to get two independent arrays?

Solutions/Answers:

Solution 1:

Use this:

var newArray = oldArray.slice();

Basically, the slice() operation clones the array and returns a reference to a new array.

Also note that:

For references, strings and numbers (and not the actual object), slice() copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.

Primitives such as strings and numbers are immutable, so changes to the string or number are impossible.

Solution 2:

In Javascript, deep-copy techniques depend on the elements in an array. Let’s start there.

Three types of elements

Elements can be: literal values, literal structures, or prototypes.

// Literal values (type1)
const booleanLiteral = true;
const numberLiteral = 1;
const stringLiteral = 'true';

// Literal structures (type2)
const arrayLiteral = [];
const objectLiteral = {};

// Prototypes (type3)
const booleanPrototype = new Bool(true);
const numberPrototype = new Number(1);
const stringPrototype = new String('true');
const arrayPrototype = new Array();
const objectPrototype = new Object(); # or "new function () {}"

From these elements we can create three types of arrays.

// 1) Array of literal-values (boolean, number, string) 
const type1 = [true, 1, "true"];

// 2) Array of literal-structures (array, object)
const type2 = [[], {}];

// 3) Array of prototype-objects (function)
const type3 = [function () {}, function () {}];

Deep copy techniques depend on the three array types

Based on the types of elements in the array, we can use various techniques to deep copy.

Javascript deep copy techniques by element types

  • Array of literal-values (type1)
    The [...myArray], myArray.splice(0), myArray.slice(), and myArray.concat() techniques can be used to deep copy arrays with literal values (boolean, number, and string) only; where the Spread operator [...myArray] has the best performance (https://measurethat.net/Benchmarks/Show/4281/0/spread-array-performance-vs-slice-splice-concat).

  • Array of literal-values (type1) and literal-structures (type2)
    The JSON.parse(JSON.stringify(myArray)) technique can be used to deep copy literal values (boolean, number, string) and literal structures (array, object), but not prototype objects.

  • All arrays (type1, type2, type3)
    The jQuery $.extend(myArray) technique can be used to deep-copy all array-types. Libraries like Underscore and Lo-dash offer similar deep-copy functions to jQuery $.extend(), yet have lower performance. More surprisingly, $.extend() has higher performance than the JSON.parse(JSON.stringify(myArray)) technique http://jsperf.com/js-deep-copy/15.
    And for those developers that shy away from third-party libraries (like jQuery), you can use the following custom function; which has higher performance than $.extend, and deep-copies all arrays.

function copy(aObject) {
  if (!aObject) {
    return aObject;
  }

  let v;
  let bObject = Array.isArray(aObject) ? [] : {};
  for (const k in aObject) {
    v = aObject[k];
    bObject[k] = (typeof v === "object") ? copy(v) : v;
  }

  return bObject;
}

So to answer the question…

Question

var arr1 = ['a','b','c'];
var arr2 = arr1;

I realized that arr2 refers to the same array as arr1, rather than a
new, independent array. How can I copy the array to get two
independent arrays?

Answer

Because arr1 is an array of literal values (boolean, number, or string), you can use any deep copy technique discussed above, where the spread operator ... has the highest performance.

// Highest performance for deep copying literal values
arr2 = [...arr1];

// Any of these techniques will deep copy literal values as well,
//   but with lower performance.
arr2 = arr1.slice();
arr2 = arr1.splice(0);
arr2 = arr1.concat();
arr2 = JSON.parse(JSON.stringify(arr1));
arr2 = $.extend(true, [], arr1); // jQuery.js needed
arr2 = _.extend(arr1); // Underscore.js needed
arr2 = _.cloneDeep(arr1); // Lo-dash.js needed
arr2 = copy(arr1); // Custom-function needed - as provided above

Solution 3:

You can use array spreads ... to copy arrays.

const itemsCopy = [...items];

Also if want to create a new array with the existing one being part of it:

var parts = ['shoulders', 'knees'];
var lyrics = ['head', ...parts, 'and', 'toes'];

Array spreads are now supported in all major browsers but if you need older support use typescript or babel and compile to ES5.

More info on spreads

Solution 4:

No jQuery needed… Working Example

var arr2 = arr1.slice()

This copys the array from the starting position 0 through the end of the array.

It is important to note that it will work as expected for primitive types (string, number, etc.), and to also explain the expected behavior for reference types…

If you have an array of Reference types, say of type Object. The array will be copied, but both of the arrays will contain references to the same Object‘s. So in this case it would seem like the array is copied by reference even though the array is actually copied.

Solution 5:

An alternative to slice is concat, which can be used in 2 ways. The first of these is perhaps more readable as the intended behaviour is very clear:

var array2 = [].concat(array1);

The second method is:

var array2 = array1.concat();

Cohen (in the comments) pointed out that this latter method has better performance.

The way this works is that the concat method creates a new array consisting of the elements in the object on which it is called followed by the elements of any arrays passed to it as arguments. So when no arguments are passed, it simply copies the array.

Lee Penkman, also in the comments, points out that if there’s a chance array1 is undefined, you can return an empty array as follows:

var array2 = [].concat(array1 || []);

Or, for the second method:

var array2 = (array1 || []).concat();

Note that you can also do this with slice: var array2 = (array1 || []).slice();.

Solution 6:

This is how I’ve done it after trying many approaches:

var newArray = JSON.parse(JSON.stringify(orgArray));

This will create a new deep copy not related to the first one (not a shallow copy).

Also this obviously will not clone events and functions, but the good thing you can do it in one line, and it can be used for any kind of object (arrays, strings, numbers, objects …)