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# syntax Overview
# # How JavaScript is introduced
1. Write code directly inside the ' Script ' tab
"' HTML
<! DOCTYPE html>
<meta charset= "Utf-8" >
<title></title>
<script type= "Text/javascript" >
Console.log (' Hello, world ');
</script>
<body>
</body>
```
2. Introduce an external JavaScript file via the ' src ' attribute of the ' script ' tag
"' HTML
<! DOCTYPE html>
<meta charset= "Utf-8" >
<title></title>
<body>
<script type= "Text/javascript" src= "./index.js" ></script>
</body>
```
# # Data type
There are 5 primitive types in JavaScript
-Numeric type (' number ')
-String type (' string ')
-Boolean value type (' Boolean ')
-NULL
-Undefined
# # variable naming rules
Naming rules:
-The first character can be any Unicode uppercase and lowercase letters, as well as a dollar sign ($) and an underscore (_).
-The second character, followed by the character, can also be used as a number.
-Cannot use reserved word as variable name
##### *javascript Reserved Words *
' Abstract '
' Boolean ' ' Break ' ' byte '
' Case ' ' catch ' ' char ' ' class ' const ' Continue '
' Debugger ' default ' delete ' do ' double '
' Else ' enum ' ' export ' ' extends '
' False ' final ' finally ' float ' for ' function '
' Goto '
' If ' ' Implements ' ' Import ' ' in ' instanceof ' int ' interface '
' 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 '
*example*
# # # Legal naming rules
' Arg0 '
' _tmp '
' $elem '
' π '
# # # Illegal naming rules
' 1a '
' 23 '
`***`
' A+b '
'-d '
' var '
' Return '
# # operator
ECMA-262 describes a set of operators that manipulate data values, including arithmetic operators such as plus and minus signs, bitwise operators, relational operators, and equality operators.
# # # unary operator
1. Increment and decrement operators (' + + '--')
2. Unary Plus and minus operators
###### when a unary plus operator is applied to a non-numeric value, the operator casts the value to a numeric type
# # # arithmetic operator
`+` `-` `*` `/` `%`
# # # logical operator
Different types of values are converted to Boolean values
##### object = ' true '
##### empty string = ' false '
##### non-empty string = ' true '
##### value 0 = ' false '
##### any non-0 value (including ' Infinity ') = ' true '
##### ' null ' = ' false '
##### ' NaN ' = ' false '
##### ' undefined ' = ' false '
1. Logical non ('! ')
The ###### logical non-operator has an exclamation mark ('! ') that can be used for any value in JS. Regardless of what data type this value is, the operator returns a Boolean value.
###### to manipulate return values for different types of values
###### 1. Returns ' false ' if the value is an object
###### 2. Returns ' true ' if the value is an empty string
###### 3. Returns ' false ' if the value is a non-empty string
###### 4. If the value is a value of ' 0 ', return ' true '
###### 5. If the value is any non-' 0 ' numeric value (including ' Infinity '), return ' false '
###### 6. Returns ' true ' if the value is ' null '
###### 7. Returns ' true ' if the value is ' NaN '
###### 8. If the value is ' undefined ', return ' true '
2. Logic and (' && ')
###### logic with two and number (' && ') indicates that there are two operands, you can manipulate any of the two values in JS. The operator returns a Boolean value when all two data values are Boolean values. If there is a value that is not a Boolean, then a Boolean value is not necessarily returned.
###### But when there is a data value that is not a Boolean value, its return results follow the following rules:
###### 1. Returns the second operand if the first operand is an object
###### 2. If the second operand is an object, the object is returned only if the value of the first operand evaluates to ' true '
###### 3. If two operands are objects, the second operand is returned
###### 4. Returns ' null ' if one of the operands is ' null '
###### 5. If one operand is ' nan ', then return ' Nan '
###### 6. If there is an operand of ' undefined ', then return ' undefined '
3. Logic or (' | | | `)
The ###### logic or operator has two vertical bar symbols (' | | | '), there are two operands, you can manipulate any of the two values in JS.
###### returns ' false ' if the two operands are Boolean, only if two operands are ' false ', otherwise ' true ' is returned.
###### If there is an operand that is not a Boolean value, the following rule is followed:
###### 1. Returns the first operand if the first operand is an object
###### 2. Returns the second operand if the evaluation result of the first operand is ' false '
###### 3. If two numbers are objects, the first operand is returned
###### 4. Returns ' null ' if all two operands are ' null '
###### 5. Returns ' Nan ' if all two operands are ' Nan '
###### 6. Returns ' undefined ' if the two operands are ' undefined '
4. Short-circuit operation
###### in ' with ' operator (' && ') and ' or ' operator (' | | | '), if the first operand is already able to determine the result of the final return, then the second operand is not evaluated
# # relational operator
The relational operators, which are less than (' < '), greater than (' > '), less than equals (' <= '), greater than or equal (' >= '), are used to compare two values and compare the same rules as we learned in the math class. These operators will return a Boolean value.
# # equality operator
Equality operator (' = = '), inequality operator ('! = '), equality operator (' = = = '), non-congruent operator ('!== ')
# # # equal and Unequal # = First conversion type again comparison
# # # # congruent and not congruent = only compare and not convert type
# # Assignment operator
The simple assignment operator is represented by the equals sign (' = '), and the function is to assign the value on the right to the variable on the left.
# # conditional operator (trinocular operator)
' Boolean_expression? True_value:false_value '
The precedence of the # # operator
| operator | Description |
| ---------------------------------------- | -------------------------- |
| ' + + '--'-'-' + ' ~ '! ' delete ' new ' typeof ' void ' | Unary operators, return data types, object creation, undefined values |
| `*` `/` `%` | Multiplication, Division, modulo |
| `+` `-` `+` | addition, subtraction, String connection |
| ' << ' >> ' >>> ' | Shift |
| ' < ' <= ' > ' >= ' instanceof ' | Less than, equal to, greater than, greater than or equal to, instanceof |
| `==` `!=` `===` `!==` | Equal, not equal, strictly equal, not strictly equal |
| ' && ' | Logic and |
| `\|\|` | Logical OR |
| `?:` | Conditions |
| `=` | Assignment, Operation Assignment |
The following table lists the JavaScript operators by priority from highest to lowest. Operators with the same precedence are evaluated in order from left to right.
# # code Comment
The part of the source code that is ignored by the JavaScript engine is called a comment, and its purpose is to interpret the code. JavaScript provides two types of annotations: one is a single-line comment, with a '//' Start, the other is a multiline comment, placed between '/* ' and ' * * '.
"' JavaScript
Single-line Comment
/*
*
* Multi-line Comment
*
**/
```
JS Syntax overview
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