If you are a C, C++, or Java programmer, then the JavaScript operators will almost all be already familiar to you. Table 4.1 summarizes the operators, and you can refer to this table for reference. In the table, the column labeled P gives the operator precedence, and the column labeled A gives the operator associativity, which can be L (lefttoright) or R (righttoleft).
If you do not already know C, C++, or Java, the sections that follow the table explain how to interpret the table and explain what each of the operators does.
P  A  Operator  Operand Type(s)  Operation Performed 

0  L  .  object, property  property access 
L  []  array, integer  array index  
L  ()  function, args  function call  
1  R  ++  number 
preorpost increment (unary) 
R    number 
preorpost decrement (unary) 

R    number 
unary minus (negation) 

R  ~  integer 
bitwise complement (unary) 

R  !  Boolean 
logical complement (unary) 

R  typeof  any  return data type (unary)  
R  new  constructor call  create new object (unary)  
R  void  any  return undefined value (unary)  
2  L  *, /, %  numbers 
multiplication, division, remainder 
3  L  +,   numbers  addition, subtraction 
L  +  strings  string concatenation  
4  L  <<  integers  left shift 
L  >>  integers 
right shift with signextension 

L  >>>  integers 
right shift with zero extension 

5  L  <, <=  numbers or strings 
less than, less than or equal 
L  >, >=  numbers or strings 
greater than, greater than or equal 

6  L  ==  primitive types 
equal (have identical values) 
L  !=  primitive types 
not equal (have different values) 

L  ==  reference types 
equal (refer to same object) 

L  !=  reference types 
not equal (refer to different objects) 

7  L  &  integers  bitwise AND 
8  L  ^  integers  bitwise XOR 
9  L    integers  bitwise OR 
10  L  &&  Booleans  logical AND 
11  L    Booleans  logical OR 
12  R  ?:  Boolean, any, any 
conditional (ternary) operator 
13  R  =  variable, any  assignment 
R 
*=, /=, %=, +=, =, <<=, >>=, >>>=, &=, ^=, = 
variable, any  assignment with operation  
14  L  ,  any  multiple evaluation 
In general, there are three types of operators. Most JavaScript operators, like the + operator that we saw in the previous section, are binary operators that combine two expressions into a single, more complex expression. That is, they operate on two operands. JavaScript also supports a number of unary operators, which convert a single expression into a single more complex expression. The  operator in the expression 3 is a unary operator which performs the operation of negation on the operand 3. Finally, JavaScript supports one ternary operator, ?:, which combines the value of three expressions into a single expression.
When constructing JavaScript expressions, you must pay attention to the data types that are being passed to operators, and to the data types that are returned. Different operators expect their operands' expressions to evaluate to values of a certain data type. For example, it is not possible to multiply strings, so the expression "a" * "b" is not legal in JavaScript. Note, however, that JavaScript tries to convert expressions to the appropriate type whenever possible, so the expression "3" * "5" is legal. Its value is the number 15, not the string "15".
Furthermore, some operators behave differently depending on the type of the operands. Most notably, the + operator adds numeric operands but concatenates string operands. And if passed one string and one number, it converts the number to a string and concatenates the two resulting strings. For example, '1' + 0 yields the string '10'.
Finally, note that operators do not always return the same type as their operands. The comparison operators (less than, equal to, greater than, etc.) take operands of various types, but when comparison expressions are evaluated, they always return a Boolean result that indicates whether the comparison is true or not. For example, the expression a < 3 returns true if the value of variable a is in fact less than 3. As we'll see, the Boolean values returned by comparison operators are used in if statements, while loops, and for loopsJavaScript statements that control the execution of a program based on the results of evaluating expressions that contain comparison operators.
In Table 4.1 the column labeled P specifies the precedence of each operator. Operator precedence controls the order in which operations are performed. Operators with a lower number in the P column are performed before those with a higher number. Somewhat confusingly, we say that operators that are performed first (with a lower P number) have higher precedence.
Consider the following expression:
w = x + y*z;
w = (x + y)*z;
In Table 4.1 the column labeled A specifies the associativity of the operator. A value of L specifies lefttoright associativity, and a value of R specifies righttoleft associativity. The associativity of an operator specifies the order in which operations of the same precedence are performed. Lefttoright associativity means that operations are performed from left to right. For example:
w = x + y + z;
w = ((x + y) + z);
x = ~~y; w = x = y = z; q = a?b:c?d:e?f:g;
x = ~((~y)); w = (x = (y = z)); q = a?b:(c?d:(e?f:g));