Missed "use before declaration" errors

[davidnormo]

🔎 Search Terms

"reference error", "Block-scoped variable used before its declaration", "ts(2448)", "Variable is used before being assigned", "ts(2454)"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries but couldn't find a related issue.

⏯ Playground Link

https://www.typescriptlang.org/play/?jsx=0#code/MYewdgzgLgBAZiEMC8MA8AVAfACjmALhhwEoUsYMSiNz4xSBuAKGYRB1LoE8SXnQkWNyLQATgEswAcxTEuyCryZA

💻 Code

const foo = <T>(fn: () => T): T => fn();

foo(() => y); // no error for reference to y here, but there will be a runtime ReferenceError

const y: string = (() => y)(); // ts will error on the reference to y here inside IIFE

🙁 Actual behavior

TS isn't catching references to variables (let, const or var) before they are defined when used as a parameter to another function call.

🙂 Expected behavior

In the example above, I'd expect it to error on the use of y in the function argument to foo.

Additional information about the issue

I discovered the issue while writing a React component with useMemo and did a silly like this:

const MyComponent = () => {
  const value: string = useMemo(() => value, []); // no ts error but there is a ReferenceError thrown at runtime
  
  return null;
}

[MartinJohns]

Duplicate of #59723 / #11498.

[typescript-bot]

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