Best way to check if a variable exists in Javascript?

Question

Well, my question is obvious,

Example:

Define a with default value undefined :

var a;

If I want to check if a var exists, I will try with:

But in this case, a does exists and a value is undefined , but in the boolean evaluation this is false .

 var a; // default value is 'undefined' if (a) { alert('a exists'); } else { alert("a don't exists") } 

I can also try with the following example: but this example generates a error.

 // var a; // a is not defined if (a) { alert('a exists'); } else { alert("a don't exists") } 

And in this example, I try with typeof . But a is defined with undefined value by default.

 var a; if (typeof a != 'undefined') { alert('a exists'); } else { alert("a don't exists") } 

And in this example

 console.log ('var a exists:', window.hasOwnProperty('a')); 

What is the best way to verify if a variable actually exists and why?

Thanks.

Answer 1

There are three different possibilities in JS for a variable.

1. Variable 'a' not at all declared;
2. Variable 'a' declared, but unassigned (undefined). Ex: var a;
3. Variable 'a' declared, but assigned with empty or null. (Possible scenario, if a variable trying to get value from back-end)
   Ex: var a=null; var a=''; var a=null; var a='';

If you are expecting a value in variable, other than 0 or false, then better to use following condition. And of course its all based on your need and if you unsure about variable or object values, always enclose them in try & catch block.

 //var a; //var a=null; //var a=''; //var a=0; //var a='value'; if (typeof a != "undefined" && a) { alert("a is defined AND a is TRUE value"); } else { alert("a not exist OR a is a FALSE value"); }