Improve the error message: "error TS1210: Invalid use of 'arguments'. Class definitions are automatically in strict mode."

[Ark-kun]

Suggestion

I suggest a better error message telling that arguments is a reserved keyword or something that cannot be used in TS in strict mode.

✅ 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

I suggest a better error message telling that arguments is a reserved keyword or something and cannot be used as variable name.

📃 Motivating Example

I'm coming from languages that allow using words like "arguments" as a variable name.

I wrote a simple line of code: let arguments = targetTask.arguments ?? {} and got a confusing error: "Invalid use of 'arguments'. Modules are automatically in strict mode.ts(1215)". It would be less confusing if it told me what it thought the arguments is. E.g. "Invalid use of reserved keyword 'arguments'" or "Invalid use of disabled builtin function 'arguments'" etc.

💻 Use Cases

P.S. Ideally I wish TS supported the name "arguments" for local variables.

[fatcerberus]

This is not a TS restriction, but a JS one. arguments is a variable that is automatically in scope inside all functions, and in strict mode you're not allowed to shadow it. Both classes and modules are, per the ECMAScript specification, automatically strict-mode code. So arguments is not technically a reserved keyword, but strict mode makes it be treated kind of like one.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode#making_eval_and_arguments_simpler

[Ark-kun]

Sure. I'm not blaming TS for the underlying issue. I only ask for the error message improvement.

[andrewbranch]

Example playground: https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8CwAoa0C2AKAlI166IApgC7RgBOA5gK4bEB2pEA3AdAL6rcpA

[DanielRosenwasser]

Maybe

Code contained in a class is evaluated in strict mode which does not allow uses of the '{0}' variable.

[fatcerberus]

On a related note I think any reference to "strict mode" might be confusing to beginners, since they might mistake it to mean the strict flag in tsconfig, which is a totally different beast. Especially anyone getting into TS with no prior exposure to JS.

[DanielRosenwasser]

Code contained in a class is evaluated in JavaScript's strict mode which does not allow this use of '{0}'.
  For more information, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode.

[Ark-kun]

@DanielRosenwasser
strict mode which does not allow this use of '{0}'

As a total beginner I wish you characterized/named '{0}'. When the error just says does not allow this use of 'arguments' I don't really know what arguments is. Is it the language feature? Does JS in strict mode not support [function] arguments? Is it the name of some feature? If it the name of some function? Is it a keyword?

Your previous wording called it variable: which does not allow uses of the '{0}' variable, which seems a bit better.

Maybe we can say something like

Code contained in a class is evaluated in JavaScript's strict mode which does not allow this use of the legacy built-in variable '{0}'. The name '{0}' is reserved in strict mode.
For more information, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode.

Basically, we need to tell the beginners: "You cannot name your variable 'arguments'" and tell experienced JS developers "You cannot use the magical 'arguments' variable in strict mode."