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If I have a variable var a, and I want to check if it is of type array I can either do

a.constructor === Array

or,

a instanceof Array

which is a better way and why ?

Avinash Sharma
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3 Answers3

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Check out Array.isArray() function:

Array.isArray(a);
madox2
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    I am not looking for other methods I am just curious that what is the difference between these two and is there any drawbacks of one of them ? – Avinash Sharma Feb 02 '16 at 12:26
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    Well, StackOverflow is a question and answer site. When you ask for "how do I know if something is an array", then people will answer that. For your more specific question, also check the linked duplicate about `instanceof` vs `constructor`. – Thilo Feb 02 '16 at 12:28
  • @AvinashSharma Yes, the drawback is that while `Array.isArray()` *always* tells you if something is an array, both `a.constructor === Array` and `a instanceof Array` will fail in some cases ([see this for details](http://web.mit.edu/jwalden/www/isArray.html)). – Frxstrem Feb 02 '16 at 12:31
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if(typeof a === 'object' && Array.isArray(a)) {
    //its array
}

UPDATE : the difference

var a = Object.create(Array.prototype);

alert(a instanceof Array);                       //TRUE
alert(a.constructor === Array);                  //TRUE  
if (typeof a === 'object' && Array.isArray(a)) { //FALSE 
    alert(true);
} 
else {
    alert(false);
}

but then you do

var a = new Array();  // or var a = [];  etc

alert(a instanceof Array);                       //TRUE
alert(a.constructor === Array);                  //TRUE  
if (typeof a === 'object' && Array.isArray(a)) { //TRUE 
    alert(true);
} 
else {
    alert(false);
}
Oxi
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-1

To return variable type of a use below

typeof(a)
ElGavilan
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Abhijeet Dashpute
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