Learn how to declare values and variables the idiomatic way in Scala — immutability first.

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Introduction

Before you build classes, objects, or APIs, you need to understand the most basic building block: variables.

Scala makes a big deal about immutability, and this starts with how you declare data.

Let’s walk through the difference between val and var — and when to use them.

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val – Immutable Value

val name = "Alice"
val age = 30

• ✅ Think of val like a final variable in Java or a const in JavaScript.
• ❌ You cannot reassign a val.

// This will cause a compilation error:
name = "Bob" // ❌ reassignment to val

• ✅ Use val by default — it makes your code safer and easier to reason about.

var – Mutable Variable

var count = 1
count = count + 1

• You can reassign var as needed.

Type Inference (and Explicit Types)

Scala can often infer the type of a value based on the assigned data:

val city = "Mumbai"   // Inferred as String
val score = 4.5       // Inferred as Double

But you can also write the type explicitly when you want to be more specific or improve readability:

val city: String = "Mumbai"
val score: Double = 4.5

This is especially useful in function signatures, public APIs, or when collaborating in a team.

Summary