Programming Fundamentals › Programming Basics

Constant

A name bound to a value that cannot be reassigned.

Also known as: constants, const, immutable binding, named constant

A constant is a name bound to a value that doesn’t change while the program runs.

MAX_RETRIES = 3          # Python has no enforcement; ALL_CAPS is a convention
TAX_RATE = 0.2

const MAX_RETRIES = 3;   // JavaScript: cannot be reassigned

static final int MAX_RETRIES = 3;

Why use them

Replace magic numbers and repeated strings with a named value:

if attempts > 3:            # why 3?
if attempts > MAX_RETRIES:  # clear, and changeable in one place

• Meaning: the name explains the number.
• One place to change: update TAX_RATE once, not in forty places.
• Safety: where the language enforces it, accidental reassignment is an error.

Caution: constant doesn’t always mean immutable

In JavaScript, const stops reassigning the variable, but an object or array can still change inside:

const config = { debug: false };
config.debug = true;        // allowed

See var, let and const and mutable vs immutable.

Habits

• Name them in UPPER_SNAKE_CASE where that’s the convention.
• Group related constants, or use an enum for fixed sets of options.
• Put configuration that changes per environment in configuration, not constants.
• Don’t over-extract: ONE = 1 adds nothing. Name values that carry meaning.