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) {
You want the typeof operator. Specifically:
if (typeof variable !== 'undefined') {
// the variable is defined
}
The operator will check if the variable is really undefined.typeof
if (typeof variable === 'undefined') {
// variable is undefined
}
The operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.typeof
However, do note that will return . We have to be careful to avoid the mistake of initializing a variable to . To be safe, this is what we could use instead:typeof null"object"null
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?=====
