spread syntax vs slice method

Question

I was trying to understand what is the difference between spread syntax vs slice method in the following approach.

suppose I want to make an actual copy of an array,

Answer 1

Those two methods aren’t actually equivalent though. Slice is a function on Array.prototype and is aware of the array’s implementation. It will create a very efficient deep copy. More importantly though, .slice() will preserve the sparseness information of your array.

In contrast, [...Array] will simply create a new array from an iterable view of your existing array. Not necessarily as efficient.

Try this:

var a = [];
a.length = 3;
console.log("slice:", a.slice());
console.log("spread:", [...a]);

With Chrome Browser developer console, I get these results:

slice: (3) [empty × 3]
spread: (3) [undefined, undefined, undefined]

If your array is particularly huge+sparse, array.slice() will be exceptionally fast. [...array] will probably hang your browser.