Mutating Data

JavaScript has somewhat annoying rules on this. Deep copies are required if you wish to avoid mutating.

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const one = [0, 1, 2];
const two = one;
two.push(3);
 
console.log(one);
console.log(two);

output:

[ 0, 1, 2, 3 ]
[ 0, 1, 2, 3 ]

Note: slice works on arrays of primitives not objects

const one = [0, 1, 2];
const two = one.slice();
two.push(3);
 
console.log(one);
console.log(two);

output:

[ 0, 1, 2 ]
[ 0, 1, 2, 3 ]

But objects will still refer to the original arrays even with slice

const one = [
    {
        name: 'kalen',
        age: 23
    },
    {
        name: 'kailyn',
        age: '22',
    }
];
 
const two = one.slice();
two[0].name = 'Patrick';
 
console.log(one);
console.log(two);

output:

[ { name: 'Patrick', age: 23 }, { name: 'kailyn', age: '22' } ]
[ { name: 'Patrick', age: 23 }, { name: 'kailyn', age: '22' } ]

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