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Ludivine A
Ludivine A

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Operator Precedence and Associativity

If you have never heard of Operator Precedence and Associativity, or if you never truly understood that concept, that article is made for you !


In algebra, you all know that multiplication and division have a higher precedence than addition and subtraction. 

With the expression 1 + 2 * 3, you must first multiply 2 * 3 and then add 1 to the result.

Well, in Javascript, this concept is also valid, it is simply called Operator Precedence.

Example

Precedence range from 1 to 19 :

  • Multiplication has a precedence of 13.
  • Addition has a precedence of 12.
  • Grouping (putting the expression between parenthesis) has the highest precedence. (19)
console.log(1 + 2 * 3);
// The multiplication is done first, the expression turns into
console.log(1 + 6)
// The addition is then evaluated, the result is 7

console.log((1 + 2) * 3);
// The grouped expression is read first, the expression turns into
console.log(3 * 3)
// The multiplication is then evaluated, the result is 9
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The order of evaluation is also influenced by the operator associativity.

Associativity is the direction in which the expression is evaluated : right to left or left to right.

Example

Assignment operators are right-associative, which means they are read from right to left :

a = b = 5
//is the same as
a = (b = 5)
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Exception

❗ Grouped expressions are not always read first.
If you use conditional evaluations, the condition will be checked first, then the expression between parenthesis will be evaluated.

a || (b * c);
//'a' is evaluated first, then (b * c) is evaluated if 'a' is false
a && (b < c);
//'a' is evaluated first, if 'a' is true (b * c) is evaluated
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