Java Script Basics (ii) Basic syntax

I. Declaration and use of variables

JavaScript is a weakly typed language with no explicit data type, and when declaring a variable, you do not need to specify the type of the variable, and the type of the variable is determined by the value assigned to the variable.

Syntax for variable declarations:

var variable name;

Example:

var num;          // declare a variable directly, not assign a value var num =   // declares a variable and assigns a value, and the type is determined by the value of the assignment. 

Attention:

　　1, JavaScript is case-sensitive, especially the name of variables, statement keywords and so on.

2, the variable can be used without declaration, such as: num=20, but this method is easy to error, it is difficult to find the wrong, so it is not recommended.

　　

Ii. Types of data

In JavaScript, a common basic data type is provided, as shown in the following table.

Data type	Meaning
Undefined	Not defined
Null	Null value
String	String
Number	Digital
Boolean	Boolean type

Attention:

　1, although the meaning of undefined and null is different, but undefined is actually derived from null, ECMAScript defines them as equal.

2. The number type can be a 32-bit integer and can also represent a 64-bit floating-point number.

Third, operator

As in Java, JavaScript also contains operators, and the commonly used operators are shown in the following table.

1, arithmetic operators: + 、-、 *,/,%, + + 、--;

2, Comparison operators:>, <, >=, <=, = =,! =

3, logical operators:&&, | |,! ；

4, assignment operator: =;

Attention:

　　1, in JavaScript string equality is more practical "= =";

2. Determine which type a variable belongs to, using the typeof () operator.

Iv. Logical Control statements

JavaScript's logical control statements are also divided into two categories: conditional structure and looping structure, and syntax is similar to java.

1. Conditional structure:

If...else ...

Switch:

Note: Unlike switch in Java, a string type can be used in JavaScript after a case.

2. Cyclic structure:

For loop:

While loop:

Do...while Cycle:

For...in Cycle:

3. Flow Control statement:

Break: Exits the switch structure, or terminates the loop.

Continue: Terminates the current loop and enters the next loop.

V. Notes

JavaScript annotations are divided into two types, like Java:

1,//indicates a single line comment

2./**/for multiple lines of comment

　　

Java Script Basics (ii) Basic syntax