Warn when regular expression is used in conditional

🔍 Search Terms

• conditional
• regular expression
• regex

✅ 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

When a regular expression literal appears in a conditional, such as:

if (/abc?/) {
    foo()
} else {
    bar()
}

It would be helpful warn that bar() is unreachable. The programmer likely meant to call /abc?/.test(...)

TypeScript could help catch this easy to make mistake

📃 Motivating Example

We recently were bitten by this in VS Code. It slipped through code review too

💻 Use Cases

Help programmers catch an easy to make mistake

No files, tests, or entry points are named. Start by locating TypeScript's conditional-expression diagnostics and existing unreachable-code checks; done means regular-expression literals in conditionals produce a warning without changing emitted JavaScript, with coverage for the shown if/else case.