Operator precedence determines the order in which operators are evaluated. Operators with higher precedence are evaluated first.
A common example:
3 + 4 * 5 // returns 23
The multiplication operator ("*") has higher precedence than the addition operator ("+") and thus will be evaluated first.
Associativity determines the order in which operators of the same precedence are processed. For example, consider an expression:
a OP b OP c
Left-associativity (left-to-right) means that it is processed as (a OP b) OP c, while right-associativity (right-to-left) means it is interpreted as a OP (b OP c). Assignment operators are right-associative, so you can write:
a = b = 5;
with the expected result that a and b get the value 5. This is because the assignment operator returns the value that it assigned. First, b is set to 5. Then the a is also set to 5, the return value of b = 5, aka right operand of the assignment.
The following table is ordered from highest (20) to lowest (0) precedence.
| Precedence | Operator type | Associativity | Individual operators |
|---|---|---|---|
| 20 | Grouping | n/a | ( … ) |
| 19 | Member Access | left-to-right | … . … |
Computed Member Access | left-to-right | … [ … ] | |
new (with argument list) | n/a | new … ( … ) | |
| 18 | Function Call | left-to-right | … ( … ) |
new (without argument list) | right-to-left | new … | |
| 17 | Postfix Increment | n/a | … ++ |
Postfix Decrement | n/a | … -- | |
| 16 | Logical NOT | right-to-left | ! … |
| Bitwise NOT | right-to-left | ~ … | |
| Unary Plus | right-to-left | + … | |
| Unary Negation | right-to-left | - … | |
| Prefix Increment | right-to-left | ++ … | |
| Prefix Decrement | right-to-left | -- … | |
| typeof | right-to-left | typeof … | |
| void | right-to-left | void … | |
| delete | right-to-left | delete … | |
| 15 | Exponentiation | right-to-left | … ** … |
| 14 | Multiplication | left-to-right | … * … |
| Division | left-to-right | … / … | |
| Remainder | left-to-right | … % … | |
| 13 | Addition | left-to-right | … + … |
| Subtraction | left-to-right | … - … | |
| 12 | Bitwise Left Shift | left-to-right | … << … |
| Bitwise Right Shift | left-to-right | … >> … | |
| Bitwise Unsigned Right Shift | left-to-right | … >>> … | |
| 11 | Less Than | left-to-right | … < … |
| Less Than Or Equal | left-to-right | … <= … | |
| Greater Than | left-to-right | … > … | |
| Greater Than Or Equal | left-to-right | … >= … | |
| in | left-to-right | … in … | |
| instanceof | left-to-right | … instanceof … | |
| 10 | Equality | left-to-right | … == … |
| Inequality | left-to-right | … != … | |
| Strict Equality | left-to-right | … === … | |
| Strict Inequality | left-to-right | … !== … | |
| 9 | Bitwise AND | left-to-right | … & … |
| 8 | Bitwise XOR | left-to-right | … ^ … |
| 7 | Bitwise OR | left-to-right | … | … |
| 6 | Logical AND | left-to-right | … && … |
| 5 | Logical OR | left-to-right | … || … |
| 4 | Conditional | right-to-left | … ? … : … |
| 3 | Assignment | right-to-left | … = … |
… += … | |||
… -= … | |||
… **= … | |||
… *= … | |||
… /= … | |||
… %= … | |||
… <<= … | |||
… >>= … | |||
… >>>= … | |||
… &= … | |||
… ^= … | |||
… |= … | |||
| 2 | yield | right-to-left | yield … |
| yield* | right-to-left | yield* … | |
| 1 | Spread | n/a |
... … |
| 0 | Comma / Sequence | left-to-right | … , … |
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence