Generic Constraints
Constraints
π¨βπΌ Sometimes you need to limit what types can be used with a generic. For
example, a function that accesses
.length only works with types that have a
length property.// β Error: Value might not have .length
function logLength<Value>(value: Value) {
console.log(value.length)
}
// β
Constrain Value to types with length
function logLength<Value extends { length: number }>(value: Value) {
console.log(value.length) // Safe!
}
logLength('hello') // β
strings have length
logLength([1, 2, 3]) // β
arrays have length
logLength(42) // β Error: number has no length
Use
extends to add constraints.π¨ Open
and:
- Create a constrained function that requires objects with an
idproperty - Use
keyofconstraint for type-safe property access (we'll cover type operators in depth in the Advanced TypeScript workshop) - Create a function with multiple constrained type parameters
This is a small preview of type operators. We'll dive deeper into these in
the Advanced TypeScript workshop.
keyof ObjectType produces a union of all property names of type ObjectType.
For example, if ObjectType is { name: string; age: number }, then
keyof ObjectType is 'name' | 'age'.ObjectType[Key] is an indexed access type - it gives you the type of
property Key on type ObjectType. If ObjectType is { name: string } and Key
is 'name', then ObjectType[Key] is string.
