TypeScript Default Type Parameter

<T = DefaultType>
<T extends ConstraintType = DefaultType>

// Valid: Required parameter 'T' precedes defaulted parameter 'U'
type Pair<T, U = boolean> = { first: T, second: U };

// Invalid: Required parameter 'U' follows defaulted parameter 'T'
// Error: Required type parameters may not follow optional type parameters.
type InvalidPair<T = string, U> = { first: T, second: U }; 

// Valid: 'string' satisfies the 'string | number' constraint
type Identifier<T extends string | number = string> = { id: T };

// Invalid: 'boolean' does not satisfy the 'string | number' constraint
// Error: Type 'boolean' does not satisfy the constraint 'string | number'.
type InvalidIdentifier<T extends string | number = boolean> = { id: T }; 

// Valid: 'U' defaults to whatever type is resolved for 'T'
type Transformer<T, U = T> = {
    input: T;
    output: U;
};

let identity: Transformer<number>; // T = number, U = number
let cast: Transformer<number, string>; // T = number, U = string