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
– gipouf, Commented Mar 31, 2013 at 20:57

metadept · Accepted Answer · 2013-03-31 20:52:24Z

34

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, 2013 at 20:52

metadept

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2 Comments

Devid Over a year ago

Is there a opposite of the operator in ?

AgBorkowski Over a year ago

logical, if (!("someKey" in someObject))

Travis J · Accepted Answer · 2013-03-31 20:51:32Z

11

jsFiddle Demo

Use hasOwnProperty like this:

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

answered Mar 31, 2013 at 20:51

Travis J

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2 Comments

Rodrigo García Over a year ago

Maybe better use hasOwnProperty as Object.prototype.hasOwnProperty.call(a, 'index') neat explanation here

Travis J Over a year ago

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.

Raphael Soares · Accepted Answer · 2021-08-01 13:35:16Z

2

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 1, 2021 at 13:35

Raphael Soares

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1 Comment

Unmitigated Over a year ago

!! is redundant here. Also, consider the arrays [0, 1, 2] and [false, false].

abahet · Accepted Answer · 2017-09-01 15:46:20Z

1

You can also use the findindex method :

var someArray = [];

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

answered Sep 1, 2017 at 15:46

abahet

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1 Comment

Prasad Shinde Over a year ago

here x is nothing but whole object inside a array. So how this would help?

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