JavaScript check if variable exists (is defined/initialized)
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?
How to check if a variable is set in Bash
(Usually) The right way
if [ -z ${var+x} ]; then echo "var is unset"; else echo "var is set to '$var'"; fi
where ${var+x}
is a parameter expansion which evaluates to nothing if var
is unset, and substitutes the string x
otherwise.
Quotes Digression
Quotes can be omitted (so we can say ${var+x}
instead of "${var+x}"
) because this syntax & usage guarantees this will only expand to something that does not require quotes (since it either expands to x
(which contains no word breaks so it needs no quotes), or to nothing (which results in [ -z ]
, which conveniently evaluates to the same value (true) that [ -z "" ]
does as well)).
However, while quotes can be safely omitted, and it was not immediately obvious to all (it wasn't even apparent to the first author of this quotes explanation who is also a major Bash coder), it would sometimes be better to write the solution with quotes as [ -z "${var+x}" ]
, at the very small possible cost of an O(1) speed penalty. The first author also added this as a comment next to the code using this solution giving the URL to this answer, which now also includes the explanation for why the quotes can be safely omitted.
(Often) The wrong way
if [ -z "$var" ]; then echo "var is blank"; else echo "var is set to '$var'"; fi
This is often wrong because it doesn't distinguish between a variable that is unset and a variable that is set to the empty string. That is to say, if var=''
, then the above solution will output "var is blank".
The distinction between unset and "set to the empty string" is essential in situations where the user has to specify an extension, or additional list of properties, and that not specifying them defaults to a non-empty value, whereas specifying the empty string should make the script use an empty extension or list of additional properties.
The distinction may not be essential in every scenario though. In those cases [ -z "$var" ]
will be just fine.
Checking if a variable exists in javascript
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 variableName
in 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.
How can I check if a variable exists (at all) in JS?
You need to use the typeof operator to see if a variable exist or no
you need to test if the variable you are concerned with is undefined of null like so :
if (typeof variable === 'undefined' || variable === null) {
// variable does not exist
}
Check if variable is defined javascript
Use typeof
if(typeof variable === "undefined"){
/* It is undefined */
}
How to check a not-defined variable in JavaScript
In JavaScript, null
is an object. There's another value for things that don't exist, undefined
. The DOM returns null
for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined
is the value used.
Second, no, there is not a direct equivalent. If you really want to check for specifically for null
, do:
if (yourvar === null) // Does not execute if yourvar is `undefined`
If you want to check if a variable exists, that can only be done with try
/catch
, since typeof
will treat an undeclared variable and a variable declared with the value of undefined
as equivalent.
But, to check if a variable is declared and is not undefined
:
if (yourvar !== undefined) // Any scope
Previously, it was necessary to use the typeof
operator to check for undefined safely, because it was possible to reassign undefined
just like a variable. The old way looked like this:
if (typeof yourvar !== 'undefined') // Any scope
The issue of undefined
being re-assignable was fixed in ECMAScript 5, which was released in 2009. You can now safely use ===
and !==
to test for undefined
without using typeof
as undefined
has been read-only for some time.
If you want to know if a member exists independent but don't care what its value is:
if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance
If you want to to know whether a variable is truthy:
if (yourvar)
Source
How can I determine if a variable is 'undefined' or 'null'?
You can use the qualities of the abstract equality operator to do this:
if (variable == null){
// your code here.
}
Because null == undefined
is true, the above code will catch both null
and undefined
.
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