Non-nullable promises shouldn't be allowable in conditions #34717

Search Terms

Promise, conditions, if, truthiness, coercsion, known-true

Related

#9041, #33178

Suggestion

Non-nullable promises should not be implicitly converted to truthy inside of conditions.

Use Cases

This helps prevent common bugs where promises aren't await-ed (see below).

Examples

async isAllowedToDoTheSuperSecretThing(user: User) {
    /* ... */
}

async doTheSuperSecretThing(ctx: RequestContext) {
  if (isAllowedToDoTheSuperSecretThing(ctx.user)) {
    /* do the super secret thing */
  }
}

should result in an error since isAllowedToDoTheSuperSecretThing returns a non-nullable promise and is never falsey.

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

This definitely falls into the camp of goal one:
Statically identify constructs that are likely to be errors.

This would be breaking but so is #33178.