Don't allow applying the ! operator to uncalled functions

[DonaldDuck313]

🔍 Search Terms

This condition will always return true since this function is always defined. Did you mean to call it instead? TS2774

✅ 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

If someFunction is a function, if(someFunction){...} gives error TS2774 because foo is always truthy and was probably intended to be called. However, !someFunction is allowed, always returning false. I suggest that !someFunction should be an error in the same way as if(someFunction){...}.

Depending on how strict you are about backwards compatibility, this could be an option that's disabled by default, because technically it would be a breaking change (since !someFunction currently compiles but wouldn't compile with my suggestion). However, in practice, I can't think of any code that works as intended and that would be broken by this suggestion, since I don't see why anyone intentionally writing !someFunction wouldn't just use the literal false instead.

📃 Motivating Example

declare function someFunction(): boolean;

if(someFunction){    //Oops, should be someFunction(), TypeScript helpfully warns me about this
    //Do something
}

if(!someFunction){    //No error, the if block will never be executed which can lead to bugs at runtime
    //Do something else
}

💻 Use Cases

1. What do you want to use this for?

   To get an error when it's obvious that I forgot to call a function (the same reason TS2774 already exists).

2. What shortcomings exist with current approaches?

   I have to run the code, see that something is wrong, and debug it. In the best of cases this is unnecessarily time consuming, in the worst of cases I won't notice that anything is wrong and the bug will go into production code.

3. What workarounds are you using in the meantime?

   Runtime debugging, I guess I don't have much choice.

[MartinJohns]

Duplicate of #45667.

[kirkwaiblinger]

@DonaldDuck313 FYI you can use typescript-eslint to find these cases; see

• https://typescript-eslint.io/rules/no-unnecessary-condition/
• https://typescript-eslint.io/rules/strict-boolean-expressions/

[typescript-bot]

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