declare <typecast> syntax

[PoolloverNathan]

Suggestion

🔍 Search Terms

is:issue declare as

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

declare <typecast> syntax that replaces the type of the identifier used in the typecast. Ideally this would work inside an if statement to only replace the part of the union narrowed to by the if statement. TS2352 should be raised if it would be raised by a normal (non-declared) typecast.

💻 Use Cases

When you know that a variable is a certain type, but Typescript doesn't (i.e. untyped libraries), you need to create a new variable and use typecasts. This creates an additional variable.

Test Cases

const x = 2;
declare x as string;
x // string

const x = RiggedMath.random(); // provides 0-0.5 or MyTwo, but is typed as () => number
if (x > 0.5) {
  declare x as RiggedMath.MyTwo;
}
x // number | MyTwo

[MartinJohns]

Duplicate of #10421. It's the exact same suggestion, just different keywords.

you need to create a new variable and use typecasts. This creates an additional variable.

You can use assertion-functions instead. This creates no new variable. Only disadvantage is that you require a function, and it has a close-to-zero runtime impact.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.