Prefix vs. postfix increment & decrement

March 14, 2025

In certain programming languages like C and JavaScript, you can use the increment (++) or decrement (--) operator to quickly:

increase or decrease a number value by 1
re-assign the updated value to a variable

In essence, the following operation:

let age = 41;
age = age + 1;
console.log(age); // 42

Can be shortened to:

let age = 41;
age++;
console.log(age); // 42

This is known as a “postfix” increment, but there’s a second variation of the same operation: the lesser-known “prefix” increment.

let age = 41;
++age;
console.log(age); // 42

At first glance, both variations appear to do precisely the same thing: they change the value by 1. But there’s a catch.

First of all, both age++ and ++age are so-called expressions, and every expression returns “something” (even if that thing is null). The “something” that gets returned here is what differentiates the two.

When using the postfix variation:

the expression returns the current value
the value held in the variable is incremented or decremented by one

let age = 41;
console.log(age++); // 41
console.log(age); // 42

Conversely, when using the prefix variation:

the value held in the variable is incremented or decremented by one
the expression returns the updated value

let age = 41;
console.log(++age); // 42
console.log(age); // 42

Notice that postfixing returns the current value before re-assigning the computed value to the variable, while prefixing returns the newly computed value.

Unless you specifically need that expression to return the updated value, it’s generally better to stick with the more common postfix variation.

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