Basics: Operators precedence

Operators are simple when used one at a time:

let b = 2
b + 2

You can combine multiple operators in the same expression:

const three = 1 + 2
three + 2 * 3

In this case you need to apply some rules to know which operation is executed first.

Take this example:

let a = 1 * 2 + ((5 / 2) % 2)

The result is 2.5, but why?

Some operations have higher precedence than the others. The precedence rules are:

  • * / % (multiplication/division/remainder)
  • + - (addition/subtraction)
  • = (assignment)

We have more than those, but it’s a good start to understand precedence rules.

Operations on the same level (like + and -) are executed in the order they are found, from left to right.

Following these rules, the operation

let a = 1 * 2 + ((5 / 2) % 2)

can be solved in this way:

let a = 1 * 2 + ((5 / 2) % 2)
let a = 2 + ((5 / 2) % 2)
let a = 2 + (2.5 % 2)
let a = 2 + 0.5
let a = 2.5

And if you feel brave, you can omit the parentheses altogether, and the result will be the same:

let a = 1 * 2 + 5 / 2 % 2 //2.5

Lessons in this unit:

0: Introduction
1: Literals, identifiers and variables
2: Comments
3: The difference between let, const and var
4: Types
5: Operators and expressions
6: Let's start with arithmetic operators
7: The assignment operator
8: ▶︎ Operators precedence
9: Strings
10: Numbers
11: Semicolons, white space and sensitivity
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