How do you check undefined ?
How do you check if a variable is undefined in JavaScript ?
Solution 1:
(typeof x === "undefined")It turns out that, checking x directly instead of passing it to a typeof operator and then doing a string match.
Solution 2:
( x === undefined )Still, this might fall into the bug where someone redefines undefined to something else.
undefined = 1 // BOOM!So, the best way to check for undefined is to use the void operator which is defined as
"The void operator evaluates the given expression and then returns undefined."Solution 3:
(x === void 0)The void operator allows inserting expressions that produce side effects into places where an expression that evaluates to undefined is desired.
Written by Boopathi Rajaa
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2 Responses
I heard that undefined is immutable in EcmaScript 5. Turns out it actually means something else than I expected: https://gist.github.com/mzgol/5356300
BTW, lodash.js uses solution 1.
@analog-nico after checking the gist everything looks fine. #1 and #2 are not really uses of "undefined" but a parameter using a JavaScript built-in object name which is not recommended but possible. You could also use parameters names like window, document or location with similar results.
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