isFinite(), isInteger(), ... as type predicates?

[titouandk]

🔍 Search Terms

Use isNumber() isNan() isFinite() as type predicates

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Number provides several functions that could be used as type predicates.

Currently, those functions provide no indication about the type of the value that passes the test:

let val: unknown;

if (Number.isInteger(val)) {
    val.toString() // error: val is of type unknown
}

📃 Motivating Example

The exemple is self-explanatory.

💻 Use Cases

What workarounds are you using in the meantime?

Manual casting to a related type.

[whzx5byb]

Duplicate of #15048

[fatcerberus]

To be clear, these can't be type predicates today because x is number is an "if and only if" relation - if the predicate is false, TS assumes x is not a number.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.