How to check for undefined in javascript?

Question

Hi

I want to make an if statement that if the value is not defined then let it go through.

I tried

if (something != "undefined")

and

if (something !== "undefined")

However it seems to go into the if statement no matter what. If it is undefined it goes through. If it is defined it goes through. Am I doing something wrong?

Answer 1

You need to use typeof.

if( typeof something != "undefined" )

Answer 2

Yes: you're comparing against a string rather than the undefined property of the global object. Instead, do

if (something !== undefined) {
   ...
}

... or even better:

if (typeof something != "undefined") {
    ...
}

Answer 3

If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined.

You can check the type of the variable:

if (typeof(something) != "undefined") ...

Sometimes you don't even have to check the type. If the value of the variable can't evaluate to false when it's set (for example if it's a function), then you can just evalue the variable. Example:

if (something) {
  something(param);
}

Answer 4

if (typeof foo == 'undefined') {
 // Do something
};

Note that strict comparison (!==) is not necessary in this case, since typeof will always return a string.

Answer 5

Provocant question: Why do you want to check if something undefined? Dont try to check every income like java-boys "if (str!=null && str ...)"

Read the "javascript all about types" tutorial from Mathias Reuter