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

Since there is no single complete and correct answer, I will try to summarize:

In general, the expression:

if (typeof(variable) != "undefined" && variable != null)

cannot be simplified, because the variable might be undeclared so omitting the typeof(variable) != "undefined" would result in ReferenceError. But, you can simplify the expression according to the context:

If the variable is global, you can simplify to:

if (window.variable != null)

If it is local, you can probably avoid situations when this variable is undeclared, and also simplify to:

if (variable != null)

If it is object property, you don't have to worry about ReferenceError:

if (obj.property != null)