The in operator checks for the existence of a property, regardless of its value, while the question at least appears to be asking how to test if the value of a variable is undefined.

I want to use JavaScript (I can also use jQuery) to do check whether a string matches the regex ^([a-z0-9]{5,})$, and get a true or false result. match() seems to check whether part of a string …

As many know, (0 == "") is true in JavaScript, but since 0 is a value and not empty or null, you may want to test for it. The following two functions return true only for undefined, null, …

Dec 4, 2014 · What would be a good way to test Javascript on a browser? Should there be a localhost in my computer? and how to create one?

Many times I'm using the string match function to know if a string matches a regular expression.

In order to understand, Let's analyze what will be the value return by the Javascript Engine when converting undefined , null and '' (An empty string also). You can directly check the same on …

In javascript, when using an if statement with multiple conditions to test for, does javascript test them all regardless, or will it bail before testing them all if it's already false? For exampl...

May 13, 2009 · This is not a duplicate of the marked duplicate. Variable resolution and object property resolution are very different things. A better duplicate is how to check if a variable …

Extending the JavaScript Array object is a really bad idea because you introduce new properties (your custom methods) into for-in loops which can break existing scripts.

Don't answer JavaScript questions with jQuery. jQuery isn't backwards compatible with JavaScript, it constantly changes breaking things posted, you save 1KB here or there by …