undefined, which indicates the absence of a value.
How to check for an undeclared variable?
typeof keyword will return
"undefined" for undeclared variables as well as for any variable containing the value
“A variable that has not been assigned a value is of type
A method or statement also returns
undefinedif the variable that is being evaluated does not have an assigned value.
Check out the following code about how to use
What’s the difference between
“Undefined is supposed to mean a variable has no value (or a property does not exist) because the programmer has not yet assigned it a value (or created the property).
undefined, as seen in the first code snippet.
Checking if a property exists on an object
The first example demonstrated checking for
undefined, whether or not those properties have been initialized.
Of course, that is assuming that the object itself has been declared.
The code example explains how to check for undeclared object properties:
As before, you may also want to check that object properties are not
null while you are checking to make sure that they are not
&& one-liner works because
undefined is falsy
As seen in the last code snippet,
undefined is a falsy value, so you can write one-liners using
&&, the logical AND operator.
I see that particular code pattern written commonly in React sites, where certain JSX elements are only included when certain props are present:
The above React component will only display the
<div> containing the image if the property is found in the props passed to the component.
Technically, the value
This falsy property of
undefined explains why some people use the
&& operator to write terse conditionals when checking for object properties.
undefined or any undeclared variable is the string
The difference between checking for undeclared variables and undeclared object properties is that undeclared variables usually result in a
ReferenceError, except when using the
Meanwhile, undeclared object properties always have the value
typeof a value is
"undefined", then it is safe to say that the value is actually
undefined — meaning it was not yet declared, declared but never assigned a value, or declared and assigned the value of
Now you know how to check for
Most of the modern languages like Ruby, Python or Java have a single null value ( nil or null), which seems a…
Differences between `undefined` and `null`
undefined means a variable has not been declared, or has been declared but has not yet been assigned a value null is an…
Checking for undefined: === versus typeof versus falsiness
There are several ways of checking whether a variable has the value undefined. This blog post explains the differences…
- Tutorial Republic has a short tutorial about checking for object properties: