What are Generic Constraints?

Generic constraints let you limit what types your generics can accept. Instead of accepting any type, you can specify "only types that have certain properties" - making your code safer and more predictable.

The kitchen analogy

Imagine a kitchen machine that can

any ingredient. Generics are like that machine - they accept any type. But for a fruit salad, you only want fruits, not random ingredients. Generic constraints are like setting the machine to

Why use constraints?

• Type safety - Prevent errors by ensuring types have required properties
• Better intellisense - TypeScript knows what properties are available
• Clear intentions - Code explicitly states what types are acceptable

Basic constraint syntax

Use extends to constrain your generics to types that have specific properties.

function displayPrice<T extends { price: number }>(item: T) - Now TypeScript guarantees that item.price exists and won't complain when you access it.

Common constraint patterns

• T extends string - Only string types
• T extends { id: number } - Only objects with an id property
• T extends keyof User - Only keys that exist on the User type

Default types

You can also set fallback types when no generic is specified.

function process<T = string>(item: T) - If no type is provided, TypeScript assumes string. Like setting your kitchen machine to make apple salad by default.

Real-world benefits

• Catch errors early - TypeScript prevents you from passing invalid types
• Flexible yet safe - Accept multiple types whilst ensuring they meet requirements
• Self-documenting - Constraints show exactly what your function expects