undeclared
A variable is undeclared if it has not been declared with an appropriate keyword (i.e. var
, let
or const
). Accessing an undeclared variable will throw a ReferenceError
.
代码实现
console.log(x); // ReferenceError: x is not defined
undefined
A variable is undefined
if it hasn’t been assigned a value. undefined
is a primitive data type in JavaScript and represents the absence of a value, intentional or otherwise.
使用样例
let x;
console.log(x); // undefined
null
A variable is assigned a value of null
like any other value. null
is also primitive data type in JavaScript and always represents the intentional absence of a value.
let x = null;
console.log(x); // null
Checking for empty states
Undeclared variables will throw an error, which makes them easy to spot and are not very common anyways. undefined
and null
can be easily spotted with a conditional as they are both falsy values. Due to that, null
and undefined
are loosely equal (==
), but not strictly equal (===
).
翻译自:https://www.30secondsofcode.org/js/s/undeclared-undefined-null