JavaScript check if variable exists (is defined/initialized)
function, initialization, javascript, undefined, variables
Solution
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?
Problem
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) { ```