You want the typeof operator. Specifically:
if (typeof variable !== 'undefined') {
// the variable is defined
}
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?
A variable is declared if accessing the variable name will not produce a ReferenceError. The expression typeof variableName !== 'undefined' will be false in only one of two cases:
- the variable is not declared (i.e., there is no
var variableNamein scope), or - the variable is declared and its value is
undefined(i.e., the variable's value is not defined)
Otherwise, the comparison evaluates to true.
If you really want to test if a variable is declared or not, you'll need to catch any ReferenceError produced by attempts to reference it:
var barIsDeclared = true;
try{ bar; }
catch(e) {
if(e.name == "ReferenceError") {
barIsDeclared = false;
}
}
If you merely want to test if a declared variable's value is neither undefined nor null, you can simply test for it:
if (variableName !== undefined && variableName !== null) { ... }
Or equivalently, with a non-strict equality check against null:
if (variableName != null) { ... }
Both your second example and your right-hand expression in the && operation tests if the value is "falsey", i.e., if it coerces to false in a boolean context. Such values include null, false, 0, and the empty string, not all of which you may want to discard.
It is important to note that 'undefined' is a perfectly valid value for a variable to hold. If you want to check if the variable exists at all,
if (window.variableName)
is a more complete check, since it is verifying that the variable has actually been defined. However, this is only useful if the variable is guaranteed to be an object! In addition, as others have pointed out, this could also return false if the value of variableName is false, 0, '', or null.
That said, that is usually not enough for our everyday purposes, since we often don't want to have an undefined value. As such, you should first check to see that the variable is defined, and then assert that it is not undefined using the typeof operator which, as Adam has pointed out, will not return undefined unless the variable truly is undefined.
if ( variableName && typeof variableName !== 'undefined' )
If a variable is declared but no value has been assigned to it, then its type would be "undefined." The same type would be assigned to variables that haven't been declared. In that case, how would we check if a variable has been declared?
