downlevel-dts rewrites .d.ts files created by any version of TypeScript so
that they work with TypeScript 3.4 or later. It does this by
converting code with new features into code that uses equivalent old
features. For example, it rewrites accessors to properties, because
TypeScript didn't support accessors in .d.ts files until 3.6:

 declare class C {
  get x(): number;
}

becomes

 declare class C {
  readonly x: number;
}

Note that not all features can be downlevelled. For example,
TypeScript 4.0 allows spreading multiple tuple type variables, at any
position in a tuple. This is not allowed in previous versions, but has
no obvious downlevel emit, so downlevel-dts doesn't attempt to do
anything. Be sure to test the output of downlevel-dts with the
appropriate version of TypeScript.

Features

Here is the list of features that are downlevelled:

Omit (3.5)

 type Less = Omit<T, K>;

becomes

 type Less = Pick<T, Exclude<keyof T, K>>;

Omit has had non-builtin implementations since TypeScript 2.2, but
became built-in in TypeScript 3.5.

Semantics

Omit is a type alias, so the downlevel should behave exactly the same.

Accessors (3.6)

TypeScript prevented accessors from being in .d.ts files until
TypeScript 3.6 because they behave very similarly to properties.
However, they behave differently with inheritance, so the distinction
can be useful.

 declare class C {
  get x(): number;
}

becomes

 declare class C {
  readonly x: number;
}

Semantics

The properties emitted downlevel can be overridden in more cases than
the original accessors, so the downlevel d.ts will be less strict. See
the TypeScript 3.7 release
notes
for more detail.

asserts assertion guards (3.7)

TypeScript 3.7 introduced the asserts keyword, which provides a way to indicate that a function will throw if a parameter doesn't meet a condition.
This allows TypeScript to understand that whatever condition such a function checks must be true for the remainder of the containing scope.

Since there is no way to model this before 3.7, such functions are downlevelled to return void:

 declare function assertIsString(val: any, msg?: string): asserts val is string;
declare function assert(val: any, msg?: string): asserts val;

becomes

 declare function assertIsString(val: any, msg?: string): void;
declare function assert(val: any, msg?: string): void;

Type-only import/export (3.8)

The downlevel emit is quite simple:

 import type { T } from 'x';

becomes

 import { T } from "x";

Semantics

The downlevel d.ts will be less strict because a class will be
constructable:

 declare class C {
}
export type { C };

becomes

 declare class C {}
export { C };

and the latter allows construction:

 import { C } from "x";
var c = new C();

type modifiers on import/export names (4.5)

The downlevel emit depends on the TypeScript target version and whether type and
value imports/exports are mixed.

An import/export declaration with only import/export names that have type
modifiers

 import { type A, type B } from "x";
export { type A, type B };

becomes:

 // TS 3.8+
import type { A, B } from "x";
export type { A, B };

// TS 3.7 or less
import { A, B } from "x";
export { A, B };

A mixed import/export declaration

 import { A, type B } from "x";
export { A, type B };

becomes:

 // TS 3.8+
import type { B } from "x";
import { A } from "x";
export type { B };
export { A };

// TS 3.7 or less
import { A, B } from "x";
export { A, B };

Semantics

When an import/export declaration has only import/export names with type
modifiers, it is emitted as a type-only import/export declaration for TS 3.8+
and as a value import/export declaration for TS 3.7 or less. The latter will be
less strict (see type-only import/export).

When type and value imports/exports are mixed, two import/export declarations
are emitted for TS 3.8+, one for type-only imports/exports and another one for
value imports/exports. For TS 3.7 or less, one value import/export declaration
is emitted which will be less strict (see
type-only import/export).

#private (3.8)

TypeScript 3.8 supports the new ECMAScript-standard #private properties in
addition to its compile-time-only private properties. Since neither
are accessible at compile-time, downlevel-dts converts #private
properties to compile-time private properties:

 declare class C {
  #private
}

It becomes:

 declare class C {
  private "#private"`
}

Semantics

The standard emit for any class with a #private property just adds a
single #private line. Similarly, a class with a private property
adds only the name of the property, but not the type. The d.ts
includes only enough information for consumers to avoid interfering
with the private property:

 class C {
  #x = 1
  private y = 2
}

emits

 declare class C {
  #private
  private y
}

which then downlevels to

 declare class C {
  private "#private";
  private y;
}

This is incorrect if your class already has a field named "#private".
But you really shouldn't do this!

The downlevel d.ts incorrectly prevents consumers from creating a
private property themselves named "#private". The consumers of the
d.ts also shouldn't do this.

export * from 'x' (3.8)

TypeScript 3.8 supports the new ECMAScript-standard export * as namespace syntax, which is just syntactic sugar for two import/export
statements:

 export * as ns from 'x';

becomes

 import * as ns_1 from "x";
export { ns_1 as ns };

Semantics

The downlevel semantics should be exactly the same as the original.

[named: number, tuple: string, ...members: boolean[]] (4.0)

TypeScript 4.0 supports naming tuple members:

 type T = [foo: number, bar: string];

becomes

 type T = [/** foo */ number, /** bar */ string];

Semantics

The downlevel semantics are exactly the same as the original, but
the TypeScript language service won't be able to show the member names.

in out T (4.7)

Typescript 4.7 supports variance annotations on type parameter declarations:

 interface State<in out T> {
    get: () => T;
    set: (value: T) => void;
}

becomes:

 interface State<T> {
  get: () => T;
  set: (value: T) => void;
}

Semantics

The downlevel .d.ts omits the variance annotations, which will change the variance in the cases where they were added because the compiler gets it wrong.

Target

Since the earliest downlevel feature is from TypeScript 3.5,
downlevel-dts targets TypeScript 3.4 by default. The downlevel target is
configurable with --to argument.

Currently, TypeScript 3.0 features like unknown are not
downlevelled, nor are there any other plans to support TypeScript 2.x.

Downlevel semantics

Usage

1. $ npm install downlevel-dts
2. $ npx downlevel-dts . ts3.4 [--to=3.4]
3. To your package.json, add

 "typesVersions": {
  "<4.0": { "*": ["ts3.4/*"] }
}

4. $ cp tsconfig.json ts3.9/tsconfig.json

These instructions are modified and simplified from the Definitely Typed ones.