Why is if (!x) not a good way to check for undefined in JavaScript?
Because !x also catches null, 0, '', false, and NaN. If you only want to check for undefined, use === undefined. Use !x only when you want to catch all falsy values.
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More FAQs in How to Check for undefined in JavaScript
Use strict equality: x === undefined. This checks both type and value. It is the canonical, safe way to check if a declared variable has the value undefined.
Use typeof. typeof an undeclared variable returns 'undefined' without throwing. For example: if (typeof someGlobal !== 'undefined') is safe for feature detection.
Use the in operator or hasOwnProperty. 'a' in obj returns true if the property exists (even if its value is undefined). obj.a returns undefined whether the property is missing or set to undefined.
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