Generic Constraints

Intermediate~22 min

Generic constraints limit type parameters to specific types using extends, ensuring type safety while maintaining flexibility.

Constraints with Extends

Output

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Constraint Syntax: Use <T extends Type> to constrain T to types that extend or implement Type.

keyof Constraint

The keyof operator creates a union of all property keys:

function getValue(obj: T, key: K): T[K] {
    return obj[key];  // Type-safe property access
}

Common Mistakes

1. No Constraint When Needed

// ✗ Error - T might not have length
function logLength(item: T) {
    console.log(item.length);  // Error!
}

// ✓ Correct - constrain to types with length
function logLength(item: T) {
    console.log(item.length);  // ✓ OK
}

2. Wrong keyof Usage

// ✗ Wrong - allows any string
function get(obj: T, key: string) {
    return obj[key];  // Unsafe!
}

// ✓ Correct - only valid keys
function get(obj: T, key: K) {
    return obj[key];  // Type-safe!
}

Best Practice: Use constraints to ensure generic types have required properties or methods.

Summary

Constraints limit type parameters with extends
keyof creates union of property keys
Ensures type safety while maintaining flexibility
Perfect for property access and object manipulation

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