what does 'in' mean in javascript if statement [duplicate]

Question

Hi i'm following a tutorial learning to use javascript.

function move(keyclick) {
if(40 in keyclick) {}
playerMove.x++;
render(); }

What does the 'in' word mean? I understand what the function is doing, but why not just use == ?

Thanks

keyclick would be an array, and in is basically doing a pseudo-code equivalent of if (keyclick[40] exists). — Marc B
– Marc B, Commented Aug 3, 2016 at 19:17
If you think in and == are interchangeable here, you don't understand what the function is doing at all. — user824425
– user824425, Commented Aug 3, 2016 at 19:20
Off Topic: @Night.owl, 40 in keyclick implies that the code managing keyclick is adding and deleting properties, wich is not good. Since the key is numeric I assume that keyclick is an Array? It would be be better to initialize this Array with 256 falses and then toggling the particular value, instead of adding and deleting properties, and checking them with in. — Thomas
– Thomas, Commented Aug 3, 2016 at 19:39

Quentin · Accepted Answer · 2016-08-03 19:19:11Z

1

The in operator is true if the string on the LHS is the name of a property that exists on the object on the RHS.

== tests if a value matches another value, which is entirely different.

answered Aug 3, 2016 at 19:19

Quentin

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Comments

4lackof · Accepted Answer · 2016-08-03 19:19:37Z

1

The in operator returns true if the specified property is in the specified object (cited from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in).

answered Aug 3, 2016 at 19:19

4lackof

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