Question

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How to check for undefined or null variable in javascript

I want to check for an defined variable in javascript. Please see the following few examples and help me which is the best method to check ‘a’ for undefined (and check for nothing else) in Javascript?

one

if(a === undefined) { … }

second

if(a === “undefined”) { … }

third

if(typeof a == “undefined”) { … }

last

if(a) { … }

Answer 1

if(typeof a == "undefined") { ... } is the best way to check if a variable is undefined.
if(a === undefined) { ... } is often the same thing, however, contrary to common belief “undefined” is NOT a keyword in javascript, and can in fact have a value assigned to it. Also, if a hasn’t been declared or initialized an error will be thrown.

if(a === "undefined") { ... } will check if a is a string with the value of “undefined”, and
if(a) { ... } will return true for all falsey values, such as null and 0.