switch case does not create separate block scope for let declarations

[duanshixuan]

🔎 Search Terms

Bug Report

🔎 Search Terms

• switch case let variable scope
• cross-case variable access
• TypeScript let block scoping in switch statements
• case branch variable visibility

⭐️ Bug Description

Unexpected scoping behavior occurs with let-declared variables in different case branches of a switch statement: a let variable declared in case 1 is accessible in case 2, and the TypeScript compiler fails to throw a compilation error for this cross-branch variable access, leading to potential runtime errors.

📝 Code Reproduction

class A {
  value: string | undefined;
}

function test(param: any, flag: number) {
  switch (flag) {
    case 1:
      // let variable declared in case 1 branch
      let jsonValue = (param as A).value;
      console.log(jsonValue);
      break;
    case 2:
      let jsonValues = (param as A).value;
      // Error: Accessing jsonValue (declared in case 1) - no TypeScript compilation error
      if (jsonValue != undefined) {
        console.log(jsonValues);
      }
      break;
    default:
      console.log("1111");
  }
}

function test1(aaa: string) {
  console.log(aaa);
}

let a = new A();
test(a, 2);

The TypeScript compiler should detect the access to the undeclared variable jsonValue in the case 2 branch and throw a compilation error (e.g., Cannot find name 'jsonValue'), adhering to the block-scoping rules of let.

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

No response

💻 Code

// Your code here
class A {
  value: string | undefined;
}

function test(param: any, flag: number) {
  switch (flag) {
    case 1:
      // let variable declared in case 1 branch
      let jsonValue = (param as A).value;
      console.log(jsonValue);
      break;
    case 2:
      let jsonValues = (param as A).value;
      // Error: Accessing jsonValue (declared in case 1) - no TypeScript compilation error
      if (jsonValue != undefined) {
        console.log(jsonValues);
      }
      break;
    default:
      console.log("1111");
  }
}

function test1(aaa: string) {
  console.log(aaa);
}

let a = new A();
test(a, 2);

🙁 Actual behavior

he TypeScript compiler does not throw any compilation errors, and the code compiles successfully. However, a runtime error ReferenceError: jsonValue is not defined is thrown when flag=2.

🙂 Expected behavior

The TypeScript compiler should detect the access to the undeclared variable jsonValue in the case 2 branch and throw a compilation error (e.g., Cannot find name 'jsonValue'), adhering to the block-scoping rules of let.

Additional information about the issue

No response

[MartinJohns]

Duplicate of #19503. Used search terms: switch let in:title