Programming Fundamentals › Programming Basics

Variable

A named reference to a value stored in memory.

Also known as: variables, identifier

A variable is a name that refers to a value, so you can use and change that value later.

age = 30                 # create age, referring to 30
age = age + 1            # now refers to 31
name = "Ana"

= means assign: take the value on the right, and let the name on the left refer to it. It isn’t the equality of mathematics.

Declaring and typing

• Python, JavaScript (let x = 1): you just assign. The value has a type, the variable doesn’t (static vs dynamic typing).
• Java, Go, TypeScript: the variable usually has a declared type, which is checked before the program runs.

let count: number = 0;

Variables hold references

Assigning one variable to another copies the value for simple types, but for lists and objects both names refer to the same thing (pass by value vs reference):

a = [1, 2]
b = a
b.append(3)
print(a)      # [1, 2, 3]

Good habits

• Choose descriptive names: retry_count, not x (naming).
• Use constants for values that never change (constants).
• Keep scope small. Declare variables near where they’re used (scope).
• Initialize before use. Reading an unassigned variable is an error or undefined.
• Don’t reuse one variable for two meanings.
• Follow the language’s naming style (snake_case, camelCase).