The fifth training assignment

1. Basic grammar

1.1 Statements and comments

javascript The execution unit of the program is line , That is, line by line execution , In general , Each line is a statement , Statement is an operation to complete a task , The statement ends with a semicolon , A semicolon indicates the end of a statement , Multiple statements end , Within one line , But when there are multiple statements on a line , Statement must end with a semicolon expression , There's no need to end with a semicolon , Usually add a semicolon after the expression ,javascript The engine treats the expression as a statement , This will produce some meaningless statements

1.2 Identifiers and keywords

An identifier is a name , Used to name variables and functions , There are specific rules and norms .

The rules ：

from unicode, Numbers , Chinese composition

1. Cannot start with a number

2. Can't be keywords and reserved words

3. Case sensitive

standard ：

1. Know what you know

2. Hump naming or underlining rules

1.3 Variable

A variable is a named memory space used to store data , Data can be stored in variables , You can also extract data from variables

1.3.1 Variable declarations

javascript It's a weak type of language , There is no need to specify the variable type when declaring variables , Direct use var Modifier to declare .

Variable declarations L:

var a;

a=10;

var b=29;

1.3.2 Variable notes

1. If you only declare and do not assign a value , Then the value of the variable is undefined

2. Variables have to be defined before they can be used , If the variable is not declared, use ,javascript Will report a mistake , Tell you that the variable is undefined .

3. It could be on the same line var Multiple variables are declared in the command .

4. If you use var Redeclare an existing variable is invalid

5. If you use var Redeclare an existing variable and assign a value , That overrides the previous value

6.javascript Is a dynamic type , Weak type language , in other words , There are no restrictions on the types of variables , Various types of values can be assigned

1.3.3 Variable Promotion

javascript The engine works by parsing the code first , Get all the declared code , And then run line by line , The result is the declaration of all variables , Will be promoted to the head of the code , This is called variable promotion

2 data type

although js It's a weakly typed language , Variables have no type, but the data itself is typed , For different types , We can do different operations .

javascript There is 6 Different types , There are five different types ：undefined,null, Boolean , Values and strings . A complex data type object

2.1undefined

undefined The type value is undefined

undefined Is a raw value representing nothing , Means that the value does not exist

appear undefined Common situation of ：

1. When declaring a variable without an initial value , The value of this variable is undefined

2. When calling a function, the function has formal parameters but no arguments , Namely undefined

3. When the function does not return a value , Default return undefined

2.2null

null Type is a data type with only one value , It's a special value null, It means null , It represents an empty object reference

2.3 Boolean type

The boolean type has two values ：true,false. It is often used to judge and loop conditions

2.4 Numerical type

Numerical types include ： Integer and floating point

1. All the figures are based on 64 Store for floating point

2. When storing numeric data, the floating-point type that can be converted into integer is automatically converted into integer

2.5 character string

use "" or '' Lead up

Use the plus sign “+” String splicing