Undefined

suggest change

Declared variable without a value will have the value undefined

var a;

console.log(a); // logs: undefined

Trying to retrieve the value of undeclared variables results in a ReferenceError. However, both the type of undeclared and unitialized variables is “undefined”:

var a;
console.log(typeof a === "undefined"); // logs: true
console.log(typeof variableDoesNotExist === "undefined"); // logs: true

Feedback about page:

Feedback:
Optional: your email if you want me to get back to you:


Declarations and assignments:
* Syntax
* Undefined

Table Of Contents
5 Declarations and assignments
11 Arrays
12 Objects
14 Classes
16 Map
17 Set
24 Loops
27 Date
29 Scope
30 AJAX
35 Cookies
41 JSON
44 Fetch
45 Modules
46 Screen
64 Console
68 Symbols
73 Modals
76 Events
86 Proxy
89 WeakMap
90 WeakSet
102 Tilde