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Why we need to copy JSON object?

Sometimes server respond with data like this:

{
    "code": 200,
    "msg": {
        "list": [
            {
                "text": "string 1",
                "value": 1
            },
            {
                "text": "string 2",
                "value": 2
            }
        ]
    }
}

And we modify the list like this:

let list = ret.msg.list;
list.push({
    text: 'string 3',
    value: 3
});
ret.msg.list === [
    {
        text: 'string 1',
        value: 1
    },
    {
        text: 'string 2',
        value: 2
    },
    {
        text: 'string 3',
        value: 3
    }
]

Now, we will find the original response modified. What if I want it not to be changed.

Why it's changed?

Because javascript passing variables by pointer, which means list above is always pointing to the array that server responded.

So when we modify list, will simultaneously modify the server response.

This will happen when we cope with arrays and js objects.

So we copy it.

let list = ret.msg.list.map(item => {
    item.text,
    item.value
});
list.push({
    text: 'string 3',
    value: 3
});
ret.msg.list === [
    {
        text: 'string 1',
        value: 1
    },
    {
        text: 'string 2',
        value: 2
    }
]

What if the respond grows more deeper?

We can use jQuery, underscore, lodash.

let list = $.extend({}, ret.msg).list;
let list = _.extend({}, ret.msg).list;
let list = _.clone(ret.msg.list, true);
let list = _.cloneDeep(ret.msg.list);

A much more simply way.

let list = JSON.parse(JSON.stringify(ret.msg.list));

But this only works for JSON objects, removes Function, undefined values, and keeps what JSON objects takes.

Difference between JSON object and js object.

Reference