Arithmetic Operators
算术运算符

Assignment Operators
赋值运算符

Comparison Operators
比较（关系）运算符

x==y returns true
x===y returns false

Logical Operators
逻辑运算符

(x < 10 && y > 1) returns true

(x==5 || y==5) returns false

!(x==y) returns true

String Operator
串符（连接作用）

A string is most often text, for example "Hello World!". To stick two or more string variables together, use the + operator.
在文字当中使用的比较多，举例来说“Hello World!”要将两个或多个字符串变量衔接在一起的话就得使用 + 符号

txt1="What a very"
txt2="nice day!"

txt3=txt1+txt2 

The variable txt3 now contains "What a verynice day!".
txt3变量现在包含“What a verynice day!”（把1和2衔接起来了）

To add a space between two string variables, insert a space into the expression, OR in one of the strings.
要给两个字符串变量中间添加空格就得在表达式里插入空格，或在其中的一个加上（空格）

txt1="What a very"
txt2="nice day!"
txt3=txt1+" "+txt2

or

txt1="What a very "
txt2="nice day!"
txt3=txt1+txt2

The variable txt3 now contains "What a very nice day!".
现在变量txt3为“What a very nice day!”

Conditional Operator
条件运算符

JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.
JS有根据条件不同给变量不同值的条件运算符

Syntax
语法

variablename=(condition)?value1:value2 

Example
例子

greeting=(visitor=="PRES")?"Dear President ":"Dear "

If the variable visitor is equal to PRES, then put the string "Dear President " in the variable named greeting. If the variable visitor is not equal to PRES, then put the string "Dear " into the variable named greeting.
如果变量visitor的值等于PRES那么greeting的值就为"Dear President "。如果不为PRES那么greeting的值就为"Dear"