Check if an array item is set in JS

Question

I've got an array

    var assoc_pagine = new Array();
    assoc_pagine["home"]=0;
    assoc_pagine["about"]=1;
    assoc_pagine["work"]=2;

I tried

    if (assoc_pagine[var] != "undefined") {

but it doesn't seem to work

I'm using jquery, I don't know if it can help

Thanks

Answer 1

Use the in keyword to test if a attribute is defined in a object

if (assoc_var in assoc_pagine)

OR

if ("home" in assoc_pagine)

---

There are quite a few issues here.

Firstly, is var supposed to a variable has the value "home", "work" or "about"? Or did you mean to inspect actual property called "var"?

If var is supposed to be a variable that has a string value, please note that var is a reserved word in JavaScript and you will need to use another name, such as assoc_var.

var assoc_var = "home";
assoc_pagine[assoc_var] // equals 0 in your example

If you meant to inspect the property called "var", then you simple need to put it inside of quotes.

assoc_pagine["var"]

Then, undefined is not the same as "undefined". You will need typeof to get the string representation of the objects type.

This is a breakdown of all the steps.

var assoc_var = "home"; 
var value = assoc_pagine[assoc_var]; // 0
var typeofValue = typeof value; // "number"

So to fix your problem

if (typeof assoc_pagine[assoc_var] != "undefined") 

---

update: As other answers have indicated, using a array is not the best sollution for this problem. Consider using a Object instead.

var assoc_pagine = new Object();
assoc_pagine["home"]=0;
assoc_pagine["about"]=1;
assoc_pagine["work"]=2;

Answer 2

var assoc_pagine = new Array();
assoc_pagine["home"]=0;

Don't use an Array for this. Arrays are for numerically-indexed lists. Just use a plain Object ({}).

What you are thinking of with the 'undefined' string is probably this:

if (typeof assoc_pagine[key]!=='undefined')

This is (more or less) the same as saying

if (assoc_pagine[key]!==undefined)

However, either way this is a bit ugly. You're dereferencing a key that may not exist (which would be an error in any more sensible language), and relying on JavaScript's weird hack of giving you the special undefined value for non-existent properties.

This also doesn't quite tell you if the property really wasn't there, or if it was there but explicitly set to the undefined value.

This is a more explicit, readable and IMO all-round better approach:

if (key in assoc_pagine)

Answer 3

var is a statement... so it's a reserved word... So just call it another way. And that's a better way of doing it (=== is better than ==)

if(typeof array[name] !== 'undefined') {
    alert("Has var");
} else {
    alert("Doesn't have var");
}

Answer 4

This is not an Array. Better declare it like this:

var assoc_pagine = {};
assoc_pagine["home"]=0;
assoc_pagine["about"]=1;
assoc_pagine["work"]=2;

or

var assoc_pagine = {
                 home:0,
                 about:1,
                 work:2
               };

To check if an object contains some label you simply do something like this:

if('work' in assoc_pagine){
   // do your thing
};

Answer 5

TLDR; The best I can come up with is this: (Depending on your use case, there are a number of ways to optimize this function.)

function arrayIndexExists(array, index){
    if ( typeof index !== 'number' && index === parseInt(index).toString()) {
        index = parseInt(index);
    } else {
        return false;//to avoid checking typeof again
    }
    return typeof index === 'number' && index % 1===0 && index >= 0 && array.hasOwnKey(index);
}

The other answer's examples get close and will work for some (probably most) purposes, but are technically quite incorrect for reasons I explain below.

Javascript arrays only use 'numerical' keys. When you set an "associative key" on an array, you are actually setting a property on that array object, not an element of that array. For example, this means that the "associative key" will not be iterated over when using Array.forEach() and will not be included when calculating Array.length. (The exception for this is strings like '0' will resolve to an element of the array, but strings like ' 0' won't.)

Additionally, checking array element or object property that doesn't exist does evaluate as undefined, but that doesn't actually tell you that the array element or object property hasn't been set yet. For example, undefined is also the result you get by calling a function that doesn't terminate with a return statement. This could lead to some strange errors and difficulty debugging code.

This can be confusing, but can be explored very easily using your browser's javascript console. (I used chrome, each comment indicates the evaluated value of the line before it.);

var foo = new Array();
foo;
//[]
foo.length;
//0
foo['bar'] = 'bar';
//"bar"
foo;
//[]
foo.length;
//0
foo.bar;
//"bar"

This shows that associative keys are not used to access elements in the array, but for properties of the object.

foo[0] = 0;
//0
foo;
//[0]
foo.length;
//1
foo[2] = undefined
//undefined
typeof foo[2]
//"undefined"
foo.length
//3

This shows that checking typeof doesn't allow you to see if an element has been set.

var foo = new Array();
//undefined
foo;
//[]
foo[0] = 0;
//0
foo['0']
//0
foo[' 0']
//undefined

This shows the exception I mentioned above and why you can't just use parseInt();

If you want to use associative arrays, you are better off using simple objects as other answers have recommended.

Answer 6

if (assoc_pagine.indexOf('home') > -1) {
   // we have home element in the assoc_pagine array
}

Mozilla indexOf

Answer 7

This worked for me

if (assoc_pagine[var] != undefined) {

instead this

if (assoc_pagine[var] != "undefined") {

Answer 8

function isset(key){
ret = false;
array_example.forEach(function(entry) {
  if( entry == key ){
    ret = true;
  }
});
return ret;
}

alert( isset("key_search") );

Answer 9

The most effective way:

if (array.indexOf(element) > -1) {
   alert('Bingooo')
}

W3Schools