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What is the difference between prefix and postfix operators?

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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Problem

The increment operator (++) adds 1 to its operand and returns a value. Similarly, the decrement operator (--) subtracts 1 from its operand and returns a value. Both of these operators can be used either prefix (++i, --i) or postfix (i++, i--).
If used prefix, the value is incremented/decremented, and the value of the expression is the updated value.
If used postfix, the value is incremented/decremented, and the value of the expression is the original value.

Solution

let i = 0;    // i = 0
let j = ++i;  // i = 1, j = 1
let k = --i;  // i = 0, k = 0

If used postfix, the value is incremented/decremented, and the value of the expression is the original value.

Code Snippets

let i = 0;    // i = 0
let j = ++i;  // i = 1, j = 1
let k = --i;  // i = 0, k = 0

let i = 0;    // i = 0
let j = i++;  // i = 1, j = 0
let k = i--;  // i = 0, k = 1

Context

From 30-seconds-of-code: prefix-postfix-operators

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