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Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))
if (elem) { // or !elem or
if (typeof elem !== 'undefined') { or
if (elem != null) { 
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Use the typeof operator to check if a variable is defined or initialized, e.g. if (typeof a !== 'undefined') {} . If the the typeof operator doesn't return a string of "undefined" , then the variable is defined.
The typeof operator will check whether a variable is defined or not. The typeof operator doesn't throw a ReferenceError exception when it is used with an undeclared variable. The typeof null will return an object. So, check for null also.
Use the typeof operator to check if a function is defined, e.g. typeof myFunction === 'function' . The typeof operator returns a string that indicates the type of a value. If the function is not defined, the typeof operator returns "undefined" and doesn't throw an error.
You want the typeof operator. Specifically:
if (typeof variable !== 'undefined') { // the variable is defined } The typeof operator will check if the variable is really undefined.
if (typeof variable === 'undefined') { // variable is undefined } The typeof operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.
However, do note that typeof null will return "object". We have to be careful to avoid the mistake of initializing a variable to null. To be safe, this is what we could use instead:
if (typeof variable === 'undefined' || variable === null) { // variable is undefined or null } For more info on using strict comparison === instead of simple equality ==, see:
Which equals operator (== vs ===) should be used in JavaScript comparisons?
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