Possible Duplicate:
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) { ... }
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.
if(a === undefined) is normally the same thing. With which method are you comparing with? Secondly is it correct to say if(typeof a == "undefined")or this is correct if(typeof(a) == "undefined")?
if(a === undefined) is normally the same thing as if(typeof a == "undefined"). if(typeof a == "undefined") and if(typeof(a) == "undefined") are exactly the same. You can use whichever version you want.
a hasn't been declared or initialised an error will be thrown. The best way to check is if (typeof a == 'undefined'). Of course that doesn't distinguish between whether a has been created and assigned a value of undefined or hasn't been created at all. But then what is the point of that distinction anyway?undefined in javascript is an actual value, so if you do a === "undefined" you're just checking if a variable that is already defined has a value of undefined assigned to it. If the variable isn't hoisted yet then you'll get a nice error.
if(a) is checking if the value of a is true or false. undefined in JS as many other values is falsey so it acts as false. Still, if the variable hasn't been declared before then this will throw an error too.
typeof a === "undefined" is the correct way of checking for a variable that might not exist yet.
typeof a === "undefined" is the cirrect way of checking?
undefined or false like the other options.a hasn't been initialised or declared, where as if (a) will.if (typeof a == "undefined") {
....
}
