JavaScript check if variable exists (is defined/initialized)
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 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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