On: Deep-copying in JavaScript

[ricardobeat]

Missing from the benchmarks is simple deep copying:

    function deepClone (obj) {
        var clone = {}
        for (var key in obj) {
            if (obj.hasOwnProperty(key)) {
                var value = obj[key]
                switch (Object.prototype.toString.call(value)) {
                    case '[object Object]': clone[key] = deepClone(obj[key]); break
                    case '[object Array]' : clone[key] = value.slice(0); break
                    default               : clone[key] = value
                }
            }
        }
        return clone
    }

This is 5-10x faster than the JSON trick in WebKit: http://jsben.ch/W0ciO

(naive implementation, but covers 90% of uses, see lodash for the full mess: https://github.com/lodash/lodash/blob/master/.internal/baseC...)

[drodgers → ricardobeat]

I believe that it's faster, but only because it's not actually deep-cloning.

This:

    value.slice(0);

is a shallow clone; it makes a new array, but doesn't clone it's contents.

[ricardobeat → drodgers]

Hence being a 'naive' implementation. You can replace that with something like `deepClone(value)` + implement copying for different types, ultimately you'll end up with code like lodash's.

[drodgers → ricardobeat]

It's not just naive though, it's wrong; it won't even give you new objects although it seems like it's meant-to:

    a = [1, {foo: "bar"}];
    b = a.slice(0);

    b[1].foo = "baz";

    console.log(a)
    > [1, {foo: "baz"}]

    console.log(b)
    > [1, {foo: "baz"}]

A naive implementation would do this:

    case '[object Array]' : clone[key] = value.map((item) => deepClone(item)); break

And would thus be slower.

[ricardobeat → drodgers]

We are in agreement here. My suggestion was to make the deepClone method also take arrays, and walk through them copying values in the for loop (significantly faster than map).

Again, I linked to a full implementation at the end; was more commenting on the lack of this particular approach, wrote this on the spot as an example.