Javascript:why the array slice method could turn nodelist or arguments to array?

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?

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

Let's play a game of follow the breadcrumbs

In the es5 spec,

Array.prototype.slice(start, end)

Let relativeStart be ToInteger(start)
ToInteger

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

Undefined transforms to NaN
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.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 9, 2013 at 1:55

Paul S.

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go-oleg · Accepted Answer · 2013-07-09 01:47:33Z

0

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

answered Jul 9, 2013 at 1:47

go-oleg

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5 Comments

HMR Over a year ago

+1 for MDN and OP is right Array.prototype.slice.call(document.querySelectorAll('a')) should not work (it does in FF) because according to the documentation the begin parameter is not optional so you should use Array.prototype.slice.call(document.querySelectorAll('a'),0) You could introduce quirks leaving the begin parameter out like with an array sort function that returns true or false, works in FF but not in Chrome or Safari.

Paul S. Over a year ago

@HMR I think you'll find you can leave out the first parameter in Chrome, too. Array.prototype.slice.call({length:2}); // [undefined × 2]

HMR Over a year ago

@PaulS. Your answer explains why it works (probably in all browsers) but I think it would be best to pass the begin parameter. The fact that it works is because of the browser implementation of converting undefined into a number but when properly implemented slice should trow an error (missing mandatory parameter). It's unlikely the implementation will change because it'll probably break a lot of scripts but you never know.

Paul S. Over a year ago

@HMR where does it say it is a "mandatory parameter" anywhere in the spec? As you can plainly see, the per-spec algorithm never says to throw an exception, unlike other algorithms such as Array.prototype.every where it has 4. If IsCallable(callbackfn) is false, throw a TypeError exception.

HMR Over a year ago

@PaulS. I thought the syntax of the documentation was somefunction(mandatory[,optional]) But I guess looking at the es5 documentation a little more they'll throw in the brackets or leave it out whenever they feel like it.

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