Bloomberg feedback for 5.8

[dragomirtitian]

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

We evaluated the 5.8 beta and RC releases and 5.8 seems to be a low impact release for us.

We are excited to adopt the new flag --erasableSyntaxOnly for the improved Editor experience for features we already avoid. A significant number of our projects already comply with the corresponding rules due to custom error checks that will happily become redundant. We remain interested in gaining the ability to use first-class enums that comply with the TS=JS+types model. For example via the erasable as enum syntax proposal.

#	Change	Affects	Release notes	Packages affected
1	Computed properties with a non literal type now appear in declarations	Declaration Emit	Yes	>1%
2	Better type checking of conditional expressions	Type Checking	No Yes	~1-2%

Computed properties with a non literal type now appear in declarations

Some declaration files changed and now preserve computed properties where index signatures would previously be created. This is an expected change and part of ongoing Isolated Declarations work.

// index.ts
export let foo = "x"
export class Bar {
    [foo]: string
}

// index.d.ts
export declare let foo: string;
export declare class Bar {
    // in 5.8
    [foo]: string;
    // in 5.7
    [x: string]: string;
}

Better checking in conditional expressions

We observed roughly a dozen new compile errors relating to improved conditional return checking.

While not directly announced the change seems related to Checked Returns for Conditional and Indexed Access Types

In TypeScript 5.7 and before, if a branch of a conditional expression returned any we would not get an error if we returned the wrong type on the other branch. Now we do, at least if the expression is in the return of a function:

declare const data: number | string | undefined;
const  result = (): number =>
    typeof data === 'string'
        ? null as any
        : data; // Error now


const  result2: number =  
    typeof data === 'string'
        ? null as any
        : data; // Still not an error ? 😕

Link

[jakebailey]

For the conditional checking one, the 5.8 RC post documents it: https://devblogs.microsoft.com/typescript/announcing-typescript-5-8-rc/#granular-checks-for-branches-in-return-expressions

(It wasn't in the beta blog.)