JavaScript (Next day)

A Grammatical composition

Case sensitive

everything in ECMAScript, including variables, function names, and operators are case-sensitive. For example,text and text represent two different variables.

Identifier

The so-called identifier refers to the name of a variable, function, property, or parameter of a function. Identifiers can be one or more characters that are combined by the following formatting rules:

1. The first character must be a letter, an underscore (_), or a dollar sign ($).

2. Other characters can be letters, underscores, dollar signs, or numbers.

3. You cannot use keywords, reserved words,true,false , and null as identifiers.

For example:mynickname,book123 , etc.

Comments

ECMAScript uses C -style annotations, including single-line comments and block-level annotations.

single-line comment

/*

* This is a multi-line

* Notes

*/

Direct Volume ( literal literal)

All Direct Quantities ( literal ) is the data value that is displayed directly in the program.

+// digital literal

' eon ' // string literal

False// Boolean literal

/JS/GI// Regular expression literals

Null// object literal

in the ECMAScript version 3 , expressions like array literals and object literals are also supported, as follows:

{x:1, Y:2}// object literal expression

[1,2,3,4,5]// array literal expression

Two Keywords and reserved words

ECMAScript-262 describes a set of keywords that are used for a specific purpose, typically to control the start or end of a statement, or to perform a specific operation. Keywords are also language-reserved and cannot be used as identifiers.

ECMAScript All keywords

Break	Else	New	Var
Case	Finally	Return	void
Catch	For	Switch	While
Continue	function	This	With
Default	If	Throw
Delete	Inch	Try
Do	instanceof	typeof

ECMAScript-262 also describes another set of reserved words that cannot be used as identifiers. Although reserved words do not have a specific purpose in JavaScript, they are most likely to be used as keywords in the future.

ECMAScript-262 all reserved words defined in version 3

Abstract	Enum	Int	Short
Boolean	Export	Interface	Static
Byte	Extends	Long	Super
Char	Final	Native	Synchronized
Class	Float	Package	Throws
Const	Goto	Private	Transient
Debugger	Implements	Protected	Volatile
Double	Import	Public

Three Variable

The ECMAScript variable is loosely typed, and the so-called loose type is used to hold any type of data. Use the var operator when defining variables (var is the key), followed by a variable name (the variable name is an identifier).

var box;

alert (box);

This statement defines the box variable, but does not initialize it (that is, it does not assign a value to the variable). At this point, the system gives it a special value --undefined(which means undefined).

var box= ' Zhang San ';

alert (box);

the so-called variable is the amount that can be changed again after initialization. ECMAScript is a weakly typed ( loosely typed ) language that can change the amount of different types at the same time. PS: Although different types of quantities can be changed, this is difficult for later maintenance, and the performance is not high, resulting in high cost! )

var boxstring = ' Zhang San ';

boxstring = 100;

alert (boxstring);

repeat using var to declare a variable, but it is an assignment operation, and does not give an error. But this kind of operation is the comparison of two, there is no need.

var box= ' Zhang San ';

var box= ' John Doe ';

There is also a variable that does not require the preceding var keyword to create a variable. This variable and var variable have a certain difference and scope, we will discuss in detail in the scope that section.

Box= ' Zhang San ';

When you want to declare multiple variables, you can do it in one or more rows.

var box= ' Zhang San '; var age=;

you can omit semicolons when each statement is in a different row. (PS: This is ECMAScript support, but definitely a very bad programming habit, remember not ).

var box= ' Zhang San '

var age= 100

Alert (Box)

You can use one statement to define multiple variables, as long as each variable ( can be initialized or not initialized ) separated by commas, for readability, each variable, preferably another row, and the second variable and the first variable are aligned (PS: These are not required ) .

var box= ' Zhang San ',

Age = 28,

Height

JavaScript (Next day)