扩散元素在这里起作用的目的是什么

Question

I'm learning react from a book and in an example the author made a component render this way

return (
        <div className="star-rating">
        {[...Array(totalStars)].map((n,i)=>
            <Star key = {i} selected={i<starSelected} onClick={()=>this.change(i+1)} />
        )}
        <p>{starsSelected} of  {totalStars} stars </p>+
    )

What purpose does the spread element serve here when initializing the array?

Answer 1

Array(totalStars) makes an array with totalStars number of empty slots. Using the spread operator coerces the empty slots into undefined , effectively filling the array with SOMETHING to be mapped over (even if it is undefined).

Just instantiating an Array with Array() doesn't put anything in it's spaces, just allocates the space for them.

Answer 2

Spread operator means when we want things to be spread in form of array like -

function ABC (...) {

console. log(...); //this will print as array [1,3,5]

}

ABC(1,3,5);

When we make call to function, we are just passing as parameters not as array. But they coverted into form of Array with spread operators.