Javascript：为什么数组切片方法可以将nodelist或参数转换为数组？

Question

I know the usage like:

Array.prototype.slice.call(document.querySelectorAll('a')) 

to turn Nodelist datatype to array without a parameter, but I read from the W3CSchool about the usage of slice , the first parameter start is required:

start Required. An integer that specifies where to start the selection (The first element has an index of 0). Use negative numbers to select from the end of an array

so without a parameter and call that method is just OK? Why this could success?

Answer 1

Let's play a game of follow the breadcrumbs

In the es5 spec,

1. Array.prototype.slice(start, end)

   Let relativeStart be ToInteger(start)

2. ToInteger

   1. Let number be the result of calling ToNumber on the input argument

3. ToNumber

   Undefined transforms to NaN

4. Backtrack ToInteger

   2. If number is NaN , return +0 .

So even though it is not explicitly stated to be optional, if start is undefined , it becomes 0 .

Answer 2

Better to use MDN than w3cschools/fools .

Yes, what you are doing is correct, see the "Array-like objects" section of:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice