Google feedback on TS 4.7-beta

This GitHub issue contains feedback on the TS 4.7-beta release from the team that is responsible for keeping Google’s internal software working with the latest version of TypeScript.

Executive summary

• Some changes to our TypeScript code are required to make it compile with TS 4.7.
• Detail sections below explain the changes to our code we expect to make to unblock the upgrade.

Impact summary

Change description	Announced	Libraries affected
Type Parameter No Longer Assignable to {} in strictNullChecks	yes	0.24%
lib/d.ts changes	yes	0.022%
TS2774 & TS2801 now fire for secondary conditions in if statements	no	0.017%
TS2731 now fires for keyof T	no	0.008%
Unclassified	-	0.014%

The Announced column indicates whether we were able to connect the observed change with a section in the TS4.7-beta announcement.

The following sections give more detailed explanations of the changes listed above.

Changes which were announced

Type Parameter No Longer Assignable to {} in strictNullChecks

We see type errors similar to the ones mentioned in the release announcement.

We’re looking into adding extends object to generic type parameters where possible and casts to any to silence the error everywhere else.

lib/d.ts changes

We support the typing improvements. Generally it’s clear what’s changing.

We observed the following breakdown within this category:

• fetch now also accepts URL in addition to RequestInfo for the first parameter

• AbortSignal has additional members reason and throwIfAborted

• ImageData has additional member colorSpace

• HTMLVideoElement has additional members requestVideoFrameCallback and cancelVideoFrameCallback

• Object.assign now rejects null/undefined for the first parameter

• Object.freeze has an additional overload, which causes some type errors like:

  interface Data { prop: Array<'a'|'b'> }
  export const DATA: Data = Object.freeze({prop: ['a']});
  //           ^^^^ Type 'Readonly<{ prop: string[]; }>' is not assignable to type 'Data'.
  

We expect to add casts to any to silence the error and apply proper fixes over time.

Changes which were not announced

TS2774 & TS2801 now fire for secondary conditions in if statements

This wasn’t specifically mentioned in the release announcement, but looks like a decent improvement. Example:

declare let flag: boolean;
declare function foo(): void;
declare function bar(): Promise<void>;
if (flag && foo) foo();
//          ^^^ This condition will always return true since this function is always defined. Did you mean to call it instead?(2774)
if (flag && bar()) {}
//          ^^^^^ This condition will always return true since this 'Promise<void>' is always defined.(2801)

We expect to add casts to any to silence the error and apply proper fixes over time.

TS2731 now fires for keyof T

This wasn’t specifically mentioned in the release announcement, but looks like a decent improvement. Example:

function foo<T>(key: keyof T) {
  console.log(`Key: "${key}"`);
  //                   ^^^ Implicit conversion of a 'symbol' to a 'string' will fail at runtime. Consider wrapping this expression in 'String(...)'.(2731)
}

We expect to add casts to any to silence the error and apply proper fixes over time.

[DanielRosenwasser]

It would be great to have this diagnostic in more situations.

So for the example that you gave

function extract<T>(obj: {}, other: Partial<T>) {
  (obj as T);
// ^^^^^^^^
// Conversion of type '{}' to type 'T' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
//     'T' could be instantiated with an arbitrary type which could be unrelated to '{}'.(2352)
}

@weswigham’s PR at https://github.com/microsoft/TypeScript/pull/48861 should allow casting between {} and T.

[frigus02]

That being said, I thought fetch() always accepted an URL? I’ve never passed anything but a string to it…

You’re right that string was always possible. It’s now also possible to pass e.g. new URL(...). fetch changed from accepting RequestInfo to RequestInfo | URL. Where type RequestInfo = Request | string;.