Avoid mutation in js

js

/*
In javascript array and object are mutable so to avoid mutation we need to use Spread Operator.
*/

// To add new element ('four') to an array.
const arr1 = [1,2,3];
const fourth = 'four';
const arr2 = [...arr1, fourth];

// Output for arr2: [1, 2, 3, "four"]
// Output for arr1: [1, 2, 3]

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// To add new key value pair (d: "D") to an object.
const obj1 = {a: "A", b: "B", c: "C"};
const obj2 = {...obj1, ...{d: "D"}}

// Output for obj2: {a: "A", b: "B", c: "C", d: "D"}