JS syntax usage

JS output statement

alert('value') The browser pops up an alert box

console.log('value') browser console printout information

prompt('info') The browser pops up an input box, and the user enters

Variables in JS

1.1 What is a variable

A variable is a fast space allocated by a program in memory to store data.

The use of variables is divided into two parts: 1. Declaration of variables 2. Assignment

declare a variable

var age; //declare a variable named age

var is the js keyword, used to declare variables (meaning variable variables).After you declare a variable with a keyword, the computer automatically allocates space for the variable.

age: is the variable name defined by the programmer, and we allocate space through the variable name.

Variable assignment aeg=18

Output: console.log(age); or alert(age)

Declaring a variable without assignment will report undefined.

Using a variable without declaring or assigning a value will report an error: tel is not defined

js can be used without declaring variables and assigning them directly.

js data type

Data types are divided into simple and complex types

Simple data types (Number, string, Boolean, Undefined,null)

Complex data type (object)

Simple Data Type Description

Simple Data Types , Description ,       Default

Number           Numeric, including integer and floating point             0

Boolean       Boolean type true false Equivalent to 1 and 0         false

String            

Undefined         var a The declared variable is not assigned a value at this time a=undefined   undefined

Null         var=null declares that the variable a is empty