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The fifth training assignment
2022-07-07 10:56:00 【m0_ sixty-three million five hundred and fifty-two thousand nin】
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
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