Announcing TypeScript 3.2 RC

Daniel Rosenwasser

Today we’re announcing TypeScript 3.2 RC, the release candidate of our next version of TypeScript.

To get started using the RC, you can get it through NuGet, or use npm with the following command:

npm install -g typescript@rc

You can also get editor support for TypeScript 3.2 by:

We have some important information below for NuGet users and Visual Studio 2015 users, so please continue reading if you use either product.

Let’s look at some of what’s coming in TypeScript 3.2!


As you might’ve guessed from the title of this section, TypeScript 3.2 introduces stricter checking for bind, call, and apply. But what does that mean?

Well in JavaScript, bind, call, and apply are methods on functions that allow us to do things like bind this and partially apply arguments, call functions with a different value for this, and call functions with an array for their arguments.

Unfortunately, in its earlier days, TypeScript lacked the power to model these functions, and bind, call, and apply were all typed to take any number of arguments and returned any. Additionally, ES2015’s arrow functions and rest/spread arguments gave us a new syntax that made it easier to express what some of these methods do – and in a more efficient way as well.

Still, demand to model these patterns in a type-safe way led us to revisit this problem recently. We realized that two features opened up the right abstractions to accurately type bind, call, and apply without any hard-coding:

  1. this parameter types from TypeScript 2.0
  2. Modeling parameter lists with tuple types from TypeScript 3.0

Combined, the two of of them can ensure our uses of bind, call, and apply are more strictly checked when we use a new flag called strictBindCallApply. When using this new flag, the methods on callable objects are described by a new global type called CallableFunction which declares stricter versions of the signatures for bind, call, and apply. Similarly, any methods on constructable (but not callable) objects are described by a new global type called NewableFunction.

As an example, we can look at how Function.prototype.apply acts under this behavior:

function foo(a: number, b: string): string {
    return a + b;

let a = foo.apply(undefined, [10]);              // error: too few argumnts
let b = foo.apply(undefined, [10, 20]);          // error: 2nd argument is a number
let c = foo.apply(undefined, [10, "hello", 30]); // error: too many arguments
let d = foo.apply(undefined, [10, "hello"]);     // okay! returns a string

Needless to say, whether you do any sophisticated metaprogramming, or you use simple patterns like binding methods in your class instances ( =, this feature can help catch a lot of bugs. For more details, you can check out the original pull request here.

Object spread on generic types

JavaScript supports a handy way of copying existing properties from an existing object into a new one called “spreads”. To spread an existing object into a new object, you define an element with three consecutive periods (...) like so:

let person = { name: "Daniel", location: "New York City" };

// My secret revealed, I have two clones!
let shallowCopyOfPerson = { ...person };
let shallowCopyOfPersonWithDifferentLocation = { ...person, location: "Seattle" };

TypeScript does a pretty good job here when it has enough information about the type. The type system closely tries to model the behavior of spreads and overwrites new properties, tries to ignore methods, etc. But unfortunately up until now it wouldn’t work with generics at all.

function merge<T, U>(x: T, y: U) {
    // Previously an error!
    return { ...x, ...y };

This was an error because we had no way to express the return type of merge. There was no syntax (nor semantics) that could express two unknown types being spread into a new one.

We could have come up with a new concept in the type system called an “object spread type”, and in fact we had a proposal for exactly that. Essentially this would be a new type operator that looks like { ...T, ...U } to reflect the syntax of an object spread. When both T and U are known, that type would flatten down to some new object type.

However, this is pretty complex and requires adding new rules to type relationships and inference. While we explored several different avenues, we recently arrived at two conclusions:

  1. For most uses of spreads in JavaScript, users were fine modeling the behavior with intersection types (i.e. Foo & Bar).
  2. Object.assign – a function that exhibits most of the behavior of spreading objects – is already modeled using intersection types, and we’ve seen very little negative feedback around that.

Given that intersections model the common cases, and that they’re relatively easy to reason about for both users and the type system, TypeScript 3.2 now permits object spreads on generics and models them using intersections:

// Returns 'T & U'
function merge<T, U>(x: T, y: U) {
    return { ...x, ...y };

// Returns '{ name: string, age: number, greeting: string } & T'
function foo<T>(obj: T) {
    let person = {
        name: "Daniel",
        age: 26

    return { ...person, greeting: "hello", ...obj };

Object rest on generic types

Object rest patterns are sort of the dual of object spreads. Instead of creating a new object with some extra/overridden properties, it creates a new object that lacks some specified properties.

let { x, y, z, } = obj;

In the above, the most intuitive way to look at this code is that rest copies over all the properties from obj apart from x, y, and z. For the same reason as above, because we didn’t have a good way to describe the type of rest when obj is generic, we didn’t support this for a while.

Here we also considered a new rest operator, but we saw we already had the facilities for describing the above: our Pick and Exclude helper types in lib.d.ts. To reiterate, basically picks off all of the properties on obj except for x, y, and z in the following example:

interface XYZ { x: any; y: any; z: any; }

function dropXYZ<T extends XYZ>(obj: T) {
    let { x, y, z, } = obj;
    return rest;

If we want to consider the properties of T (i.e. keyof T) except for x, y, and z, we can write Exclude<keyof T, "x" | "y" | "z">. We then want to pick those properties back off of the original type T, which gives us

Pick<T, Exclude<keyof T, "x" | "y" | "z">>`.

While it’s not the most beautiful type (hey, I’m no George Clooney myself), we can wrap it in a helper type like DropXYZ:

interface XYZ { x: any; y: any; z: any; }

type DropXYZ<T> = Pick<T, Exclude<keyof T, keyof XYZ>>;

function dropXYZ<T extends XYZ>(obj: T): DropXYZ<T> {
    let { x, y, z, } = obj;
    return rest;

BigInt Support

BigInts are part of an upcoming proposal in ECMAScript that allow us to model theoretically arbitrarily large integers. TypeScript 3.2 brings type-checking for BigInts, as well as support for emitting BigInt literals when targeting esnext.

BigInt support in TypeScript introduces a new primitive type called the bigint (all lowercase). You can get a bigint by calling the BigInt() function or by writing out a BigInt literal by adding an n to the end of any integer numeric literal:

let foo: bigint = BigInt(100); // the BigInt function
let bar: bigint = 100n;        // a BigInt literal

// *Slaps roof of fibonacci function*
// This bad boy returns ints that are *so* big!
function fibonacci(n: bigint) {
    let result = 1n;
    for (let last = 0n, i = 0n; i < n; i++) {
        const current = result;
        result += last;
        last = current;
    return result;


While you might imagine close interaction between number and bigint, the two are separate domains.

declare let foo: number;
declare let bar: bigint;

foo = bar; // error: Type 'bigint' is not assignable to type 'number'.
bar = foo; // error: Type 'number' is not assignable to type 'bigint'.

As specified in ECMAScript, mixing numbers and bigints in arithmetic operations is an error. You’ll have to explicitly convert values to BigInts.

console.log(3.141592 * 10000n);     // error
console.log(3145 * 10n);            // error
console.log(BigInt(3145) * 10n);    // okay!

Also important to note is that bigints produce a new string when using the typeof operator: the string "bigint". Thus, TypeScript correctly narrows using typeof as you’d expect.

function whatKindOfNumberIsIt(x: number | bigint) {
    if (typeof x === "bigint") {
        console.log("'x' is a bigint!");
    else {
        console.log("'x' is a floating-point number");

We’d like to extend a huge thanks to Caleb Sander for all the work on this feature. We’re grateful for the contribution, and we’re sure our users are too!


As we mentioned, BigInt support is only available for the esnext target. It may not be obvious, but because BigInts have different behavior for mathematical operators like +, -, *, etc., providing functionality for older targets where the feature doesn’t exist (like es2017 and below) would involve rewriting each of these operations. TypeScript would need to dispatch to the correct behavior depending on the type, and so every addition, string concatenation, multiplication, etc. would involve a function call.

For that reason, we have no immediate plans to provide downleveling support. On the bright side, Node.js 11 and newer versions of Chrome already support this feature, so you’ll be able to use BigInts there when targeting esnext.

Certain targets may include a polyfill or BigInt-like runtime object. For those purposes you may want to add esnext.bigint to the lib setting in your compiler options.

Breaking changes and deprecations

JSX resolution changes

Our logic for resolving JSX invocations has been unified with our logic for resolving function calls. While this has simplified the compiler codebase and improved some use-cases, there may be some differences which we may need to reconcile. These changes are likely unintentional so they are not breaking changes per se, but upgraders should note take note of any issues they encounter and report them.

lib.d.ts changes

More specific types

Certain parameters no longer accept null, or now accept more specific types as per the corresponding specifications that describe the DOM.

More platform-specific deprecations

Certain properties that are WebKit-specific have been deprecated. They are likely to be removed in a new version.

wheelDelta and friends have been removed.

wheelDeltaX, wheelDelta, and wheelDeltaZ have all been removed as they are deprecated properties on WheelEvents.

As a solution, you can use deltaX, deltaY, and deltaZ instead. If older runtimes are a concern, you can include a file called legacy.d.ts in your project and write the following in it:

// legacy.d.ts

interface WheelEvent {
     readonly wheelDelta: number;
     readonly wheelDeltaX: number;
     readonly wheelDeltaZ: number;

A note for NuGet and Visual Studio 2015

We have some changes coming in TypeScript 3.2 for NuGet and VS2015 users.

First, TypeScript 3.2 and future releases will only ship an MSBuild package, and not a standalone compiler package. Second, while our NuGet packages previously shipped with the Chakra JavaScript engine to run the compiler, the MSBuild package now depends on a globally invokable version of Node.js to be present. While machines with newer versions of Visual Studio 2017 (versions 15.8 and above) will not be impacted, testing/CI machines, users with Visual Studio 2015, and users of Visual Studio 2017 15.7 and below will need to install Node.js and will likely see a message like the following:

The build task could not find node.exe which is required to run the TypeScript compiler. Please install Node and ensure that the system path contains its location.

Lastly, TypeScript 3.2 will be the last TypeScript release with editor support for Visual Studio 2015 users. To stay current with TypeScript, we recommend upgrading to Visual Studio 2017 for the latest editing experience.

What’s next

We’ve actually got plenty more coming up in TypeScript 3.2, and you can track the list of major features we’ve been working on our Roadmap. We hope you give TypeScript 3.2 RC a try and give us some feedback on your experience. In the meanwhile, we’ll be listening to the community and polishing the 3.2 release over the next 2 weeks. Happy hacking!

– Daniel Rosenwasser and the TypeScript team


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