In javascript whats the difference between
undefined == variable and variable == undefined are both of them same? and how different will it be if i do undefined === variable or typeof variable == 'undefined'?
Can someone help me out
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All are the same. If you want to check if a variable has not been defined use "variable=== undefined". If you want to check if a variable contains data use "if (variable)". This is the same as "if (variable !== undefined && variable !== null)". You should always use === instead of ==. With == you are implicitly doing a type conversion, e.g. 1 == "1" = true, 1 === "1" = false.Gabriel Llamas– Gabriel Llamas2012-03-23 07:45:00 +00:00Commented Mar 23, 2012 at 7:45
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3 Answers
Don't use undefined to test for an undefined variable, use the typeof operator instead!
undefined isn't a javascript keyword, it's just a variable name. If someone writes var undefined = true globally anywhere into the code, all your comparisons will act unexpected.
You should consider using something like JSLINT or JSHINT to avoid these type of errors in your javascript code.
Apart from that, I would always write the compared parameter first, as that's the way I read it.
That is why If the variable foo is undefined than should be written as if (typeof foo === "undefined")
I don't remember the name for this pattern, but I'm quite sure there is one :)
Comments
There is no difference in meaning when it comes to different order of parts you mentioned.
=== is strict comparison, == is not. For example undefined == false is true, but undefined === false is not. Checking the undefined type is similar to strict comparison in this case.
Comments
undefined == variable and variable == undefined are the same.
but i recomment undefined == variable to prevent strange behalves if you miss a = (variable = undefined).
undefined === variable and typeof variable == 'undefined' should also be the same.
2 Comments
Tadeck
I disagree with your recommendation. Order is okay, but in some cases it is possible to override original value of
undefined (although in newer ES5 makes this variable read-only).
kirilloid
Yep. There's a recommendation of writing code this way, but
undefined could be rewritten in js, hence it doesn't work in this case. And overwriting undefined would lead to even more strange behavior. I prefer the option with typeof.