How to Clone an Array in Javascript?
- Time:2020-09-17 14:26:24
- Class:Weblog
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In Javascript, the arrays are passed by reference, and sometimes we want to clone an array. We cannot use the simple = operator as the reference is assigned.
1 2 3 4 | var arr = [1, 2, 3]; var newArray = arr; // true, as they both point to the same memory location console.log(arr === newArray); |
var arr = [1, 2, 3]; var newArray = arr; // true, as they both point to the same memory location console.log(arr === newArray);
We can also use Array.from, the ES6 syntax sugar – triple dot spreader …, or the old fashion way .slice() to return a new copy of the array. Let’s see the following examples.
1 2 3 4 5 6 7 8 9 10 11 12 13 | var arr = [1, 2, 3, 4]; var newArr0 = arr; var newArr1 = arr.slice(); var newArr2 = [...arr]; var newArr3 = Array.from(arr); // true console.log(arr === newArr0); // all below are showing false as new arrays are cloned console.log(arr === newArr1); console.log(arr === newArr2); console.log(arr === newArr3); |
var arr = [1, 2, 3, 4]; var newArr0 = arr; var newArr1 = arr.slice(); var newArr2 = [...arr]; var newArr3 = Array.from(arr); // true console.log(arr === newArr0); // all below are showing false as new arrays are cloned console.log(arr === newArr1); console.log(arr === newArr2); console.log(arr === newArr3);
Is that it? How about multidimensional arrays? Are the above methods supposed to do their jobs?
1 2 3 4 5 6 7 8 9 10 11 12 13 | var arr = [[1, 2], [3, 4]]; var newArr1 = arr.slice(); var newArr2 = [...arr]; var newArr3 = Array.from(arr); newArr1[0][0] = 'a'; console.log(arr[0][0]); // prints a newArr2[0][0] = 'b'; console.log(arr[0][0]); // prints b newArr3[0][0] = 'c'; console.log(arr[0][0]); // prints c |
var arr = [[1, 2], [3, 4]]; var newArr1 = arr.slice(); var newArr2 = [...arr]; var newArr3 = Array.from(arr); newArr1[0][0] = 'a'; console.log(arr[0][0]); // prints a newArr2[0][0] = 'b'; console.log(arr[0][0]); // prints b newArr3[0][0] = 'c'; console.log(arr[0][0]); // prints c
It turns out that all the above methods are providing the shallow copies – where only the first dimensions are copied by values and the subsequent dimensions are still copying the references. Therefore, if you change the values in the above 2 dimensional ‘cloned’ arrays, the original arrays are impacted even those three ‘cloned’ arrays are of different memory locations!
The deep clone (or deep copy) in Javascript, can be done via a custom deep copy function, as detailed in: How to Clone Variables (The Clone Function) in Javascript?
–EOF (The Ultimate Computing & Technology Blog) —
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