How to check for an undefined or null variable in JavaScript?

Question

We are frequently using the following code pattern in our JavaScript code

if (typeof(some_variable) != \'undefined\' && some_variable != null)
{
             


        

Answer 1

You can just check if the variable has a value or not. Meaning,

if( myVariable ) {
//mayVariable is not :
//null
//undefined
//NaN
//empty string ("")
//0
//false

}

If you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. e.g.

if( typeof myVariable !== 'undefined' ) {
    // myVariable will get resolved and it is defined
}