Why does Array.prototype.includes.bind behave unexpectedly?

Question

Given existing googleable knowledge of function.bind and such, and the following control code:

console.log( [1, 2, 3].includes(2) ) // --> `true` 
console.log( [null].map(String.prototype.toUpperCase, "abc") ) // --> `["ABC"]`  
console.log( Array.prototype.includes.bind([3, 4, 5])(1) ) //--> `false`  
console.log( Array.prototype.includes.bind([3, 4, 5])(4) ) // -- > `true`

_ is underscorejs, a generic utility wrapper/polyfill thing

[1, 2, 3].some(_.prototype.includes.bind(_([3, 4, 5]))) // --> `true`

But, why does this code yield this unexpected result?

console.log(
  [1,2,3].some(Array.prototype.includes.bind([3,4,5])) // --> `false`
)

Edit: I know this code is bad form in its literal form, but this is a POC, the real implementation will differ (and cannot use arrow functions, thanks IE)

How do I create a runnable stack snippet?

adiga
– adiga

2019-07-30 12:52:08 +00:00
Commented Jul 30, 2019 at 12:52 — adiga
– adiga, Commented Jul 30, 2019 at 12:52

adiga · Accepted Answer · 2019-07-30 12:44:06Z

3

This is the syntax for includes:

arr.includes(valueToFind, [fromIndex])

There is an optional second parameter fromIndex.

fromIndex: The position in this array at which to begin searching for valueToFind

So, your code becomes something like this:

[1,2,3].some((a,index) => Array.prototype.includes.bind([3,4,5])(a, index))
// OR
[1,2,3].some((a, index) => [3,4,5].includes(a, index))

It is looking for 3 starting from the index = 2.

So, if you were to change it like this, it will return true

console.log(
  // looks for from index=0
  [3,2,1].some(Array.prototype.includes.bind([5,4,3]))
)

edited Jul 30, 2019 at 12:44

answered Jul 30, 2019 at 12:33

adiga

35.4k9 gold badges65 silver badges88 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Marnes Over a year ago

ah dangit, you and @Grillparzer are right, the some index variable is passed into the includes fromIndex, breaking the concept. It works with underscore because it doesn't use a second variable. Thanks for pointing it out, I was overlooking this

Grillparzer · Accepted Answer · 2019-07-30 12:37:28Z

2

What you may not be expecting is that .some() invokes the bound function with three parameters, and not just the one that contains the value.

You can check this with [1,2,3].some(console.log)

Array.includes accepts two parameters, one for the value and an optional one that offsets the start location.

A solution would be:

[3,2,1].some(n => Array.prototype.includes.bind([5,4,3])(n))

answered Jul 30, 2019 at 12:37

Grillparzer

4173 silver badges9 bronze badges

Collectives™ on Stack Overflow

Why does Array.prototype.includes.bind behave unexpectedly?

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related