Copy array by value

Solution 1:

Use this:

let oldArray = [1, 2, 3, 4, 5];

let newArray = oldArray.slice();

console.log({newArray});

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.

• 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