Undefined of JavaScript data types

The data types in the Javascrip are divided into the original data type (primitive type) and the object data type (objects type).

Raw data type

The original data types are: Number, string, Boolean, null, undefined.

Object data type

An object data type is an object type in which data types in JavaScript, except numbers, strings, and Boolean values, are objects.

A brief introduction to the data types in JavaScript can be found in: http://www.easy-dis.net/?p=161, which is not described in detail here. Here is the main talk about undefined.

Undefined is a data type with only one value, which is ' undefined ', and the value of this variable is undefined when declaring a variable with VAR but not initializing it. Such as

var num;alert (num);

This will output undefined because the variable that is declared with Var is not initialized, and the result outputs a undefined value.

Undefined of JavaScript data types