Extends Constraints

In 4.7 TypeScript added support for inlining an extends clause in a conditional type. This can help reduce the complexity of conditional types. If you're new to conditional types see: example:conditional-types As an example, here is a 4.6 conditional type which looks at the return value of a function and only returns a type if the return type is a string. prettier-ignore 

type ReturnTypeOnlyStrings47 =
  T extends (...args: any[]) => 
    infer R ? (R extends string ? R : never) : never;
// It's essentially two if statements, one for the return type and one then to check if the return type is a string. In 4.7 this can be done inside one statement. prettier-ignore 
type ReturnType2 =
  T extends (...args: any[]) =>
    (infer R extends string) ? R : never;
// It's a little like being able to use an && inside the conditional type check, which makes code a bit simpler and more readable.