Intersection Types

Intermediate~18 min

Intersection types combine multiple types into one using AND logic. The result must have ALL properties from all types.

Basic Intersection Types

Output

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Intersection Syntax: Use the ampersand & to combine types: type C = A & B

When to Use Intersection Types

Mixins: Combine multiple behaviors
Composition: Build complex types from simple ones
Extending types: Add properties to existing types
Multiple constraints: Require all properties

Intersection vs Union

Feature	Intersection (&)	Union (\|)
Logic	AND - must have ALL	OR - can be ANY
Properties	Combined from all types	Only common properties
Use case	Composition, mixins	Flexible parameters
Example	`A & B` has all from A and B	`A \| B` is either A or B

Common Mistakes

1. Conflicting Property Types

type A = { value: string };
type B = { value: number };

type C = A & B;  // value is never (impossible!)

// ✓ Better - compatible types
type A = { name: string };
type B = { age: number };
type C = A & B;  // { name: string; age: number }

2. Confusing & with |

// ✗ Wrong - using union when intersection needed
function merge(a: Named | Aged) {
    // Can't access both name and age
}

// ✓ Correct - intersection requires both
function merge(a: Named & Aged) {
    console.log(a.name, a.age);  // ✓ OK
}

Best Practice: Use intersection types for composition. They're perfect for combining multiple interfaces or type aliases.

Summary

Intersection types use & for AND logic
Result must have ALL properties from all types
Perfect for composition and mixins
Avoid conflicting property types

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