How to check if an array index exist?

Question

I'm trying to check whether an array index exist in TypeScript, by the following way (Just for example):

var someArray = [];

// Fill the array with data

if ("index" in someArray) {
   // Do something
}

However, i'm getting the following compilation error:

The in operator requires the left operand to be of type Any or the String primitive type, and the right operand to be of type Any or an object type

Anybody knows why is that? as far as I know, what I'm trying to do is completely legal by JS.

Thanks.

a string. I guess that I'll do as SLaks said, i thought that typescript arrays can be used a associative arrays as well. — gipouf, Mar 31 '13 at 20:57

score 33 · Accepted Answer · answered Mar 31 '13 at 20:52

33

As the comments indicated, you're mixing up arrays and objects. An array can be accessed by numerical indices, while an object can be accessed by string keys. Example:

var someObject = {"someKey":"Some value in object"};

if ("someKey" in someObject) {
    //do stuff with someObject["someKey"]
}

var someArray = ["Some entry in array"];

if (someArray.indexOf("Some entry in array") > -1) {
    //do stuff with array
}

answered Mar 31 '13 at 20:52

metadept

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1

Is there a opposite of the operator in ? – Devid Sep 25 '17 at 13:23
1

logical, if (!("someKey" in someObject)) – AgBorkowski Jan 18 '18 at 17:16

score 11 · Answer 2 · answered Mar 31 '13 at 20:51

11

jsFiddle Demo

Use hasOwnProperty like this:

var a = [];
if( a.hasOwnProperty("index") ){
 /* do something */  
}

answered Mar 31 '13 at 20:51

Travis J

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Maybe better use hasOwnProperty as `Object.prototype.hasOwnProperty.call(a, 'index')` neat explanation [here](https://stackoverflow.com/a/12017703/3848267) – Rodrigo García Aug 13 '18 at 19:43
It's true, there are ways that the hasOwnProperty could have been abused. If you are making some sort of largely portable library, it may make sense to cover all bases like that. In general though, this works just fine. – Travis J Aug 13 '18 at 20:29

score 3 · Answer 3 · answered Aug 01 '21 at 13:35

3

One line validation. The simplest way.

if(!!currentData[index]){
  // do something
}

Outputs

var testArray = ["a","b","c"]

testArray[5]; //output => undefined
testArray[1]; //output => "b"

!!testArray[5]; //output => false
!!testArray[1]; //output => true

answered Aug 01 '21 at 13:35

Raphael Soares

482
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8

`!!` is redundant here. Also, consider the arrays `[0, 1, 2]` and `[false, false]`. – Unmitigated Dec 15 '21 at 14:01

score 2 · Answer 4 · answered Sep 01 '17 at 15:46

2

You can also use the findindex method :

var someArray = [];

if( someArray.findIndex(x => x === "index") >= 0) {
    // foud someArray element equals to "index"
}

answered Sep 01 '17 at 15:46

abahet

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here x is nothing but whole object inside a array. So how this would help? – Prasad Shinde Jun 25 '18 at 10:00

How to check if an array index exist?

4 Answers4