Declare variable keywords

Variables are used to store data

You need to declare variables first , Can be used

There are three main keywords used to declare variables ：let、const、var

var The key words are the old version （ES5）JS Used by the , Now it's out

let

let Is the keyword used to declare variables in the new version ,let The declared variables are ordinary variables

usage ：

let a = 1

//  perhaps 

let a

a = 1

The above uses let Declare a variable , A variable called a, The value is 1

Use of variables

Proper use ：

//  Declare variables first 【 This is an annotation. 】
let a = 1 //  Assign values when declaring 

//  Use variables after 
alert(a) //  The browser will pop up here ：1

The above code can also

let a // let Variables can be declared without assignment 

a = 1 //  The variable a Assign a value 

alert(a) //  Popup ：1

Incorrect usage ：

//  Use variables here first 
alert(a) //  An error will be reported here 
//  Make a statement after , Wrong usage , Because in use , Variable is not declared , So there's an error 
let a = 1

Conclusion ：

Variables must be declared before using ！！！

First use , An error will be reported after the statement

Declaration of variables

In the same environment （ Within the same scope ） All variable names cannot conflict , Otherwise, the report will be wrong

let a = 1

//  Here are other codes 

let a = 1 //  An error will be reported here 

Conclusion ：

Variables cannot be declared repeatedly within the same scope

The concept of scope will be explained in the following chapters

Change the variable value

let The value of the declared variable can be changed

let a = 1
alert(a) //  Pop up here ：1

a = 2 //  The variable... Has been changed here a The value of is 2, Use later a when , The value is a
alert(a) //  Pop up here ：2

Conclusion ：

When changing the value of a declared variable , Just assign a value to the variable directly , There is no need to write a declaration keyword

const

const The declared variable is Constant , Instead of Common variables

Constant ： A constant is a special variable , The value of this variable must be set at the time of declaration , And cannot be changed later .

const a = 1 //  declare constant a, The value is 1

a = 2 //  Modifying the value of constant here is an illegal operation , So there's an error 

besides ,const The properties of constants are similar to let Variable consistency .

Keyword reserved word list

abstract	arguments	boolean	break	byte
case	catch	char	class*	const
continue	debugger	default	delete	do
double	else	enum*	eval	export*
extends*	false	final	finally	float
for	function	goto	if	implements
import*	in	instanceof	int	interface
let	long	native	new	null
package	private	protected	public	return
short	static	super*	switch	synchronized
this	throw	throws	transient	true
try	typeof	var	void	volatile
while	with	yield

Object and method

Array	Date	eval	function	hasOwnProperty
Infinity	isFinite	isNaN	isPrototypeOf	length
Math	NaN	name	Number	Object
prototype	String	toString	undefined	valueOf

window Keyword under object

alert	all	anchor	anchors	area
assign	blur	button	checkbox	clearInterval
clearTimeout	clientInformation	close	closed	confirm
constructor	crypto	decodeURI	decodeURIComponent	defaultStatus
document	element	elements	embed	embeds
encodeURI	encodeURIComponent	escape	event	fileUpload
focus	form	forms	frame	innerHeight
innerWidth	layer	layers	link	location
mimeTypes	navigate	navigator	frames	frameRate
hidden	history	image	images	offscreenBuffering
open	opener	option	outerHeight	outerWidth
packages	pageXOffset	pageYOffset	parent	parseFloat
parseInt	password	pkcs11	plugin	prompt
propertyIsEnum	radio	reset	screenX	screenY
scroll	secure	select	self	setInterval
setTimeout	status	submit	taint	text
textarea	top	unescape	untaint	window

It should be noted that name This word , Not a keyword or reserved word , In most browsers and situations, it can be used as a variable , However, there may be very few and special cases of abnormality
| textarea | top | unescape | untaint | window |

It should be noted that name This word , Not a keyword or reserved word , In most browsers and situations, it can be used as a variable , However, there may be very few and special cases of abnormality