18. How do you redeclare variables in a switch block without an error

In JavaScript, redeclaring variables inside a switch block using let or const can lead to a "Identifier has already been declared" error because case blocks share the same scope within the switch statement.

Solution: Use a Block Scope { }

To avoid this issue, wrap each case block in a separate block { } to create a new scope:

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switch (value) {
  case 1: {
    let x = 10; // Declared inside a block, no conflict
    console.log(x);
    break;
  }
  case 2: {
    let x = 20; // This is a separate block, so no redeclaration error
    console.log(x);
    break;
  }
  default:
    console.log("Default case");
}

Why Does This Work?

• { } creates a new block scope for each case, preventing variable conflicts.
• Without { }, let x would be in the same scope across multiple case blocks, causing a redeclaration error.

Would you like more details on var, let, and const differences in this context?

Yes, here are a few alternative ways to avoid the "Identifier has already been declared" error in a switch block:

1. Use var Instead of let (Not Recommended)

Since var is function-scoped, it won’t cause a redeclaration error. However, this can lead to unexpected behavior due to hoisting.

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switch (value) {
  case 1:
    var x = 10; // No block scope issue, but can cause unintended side effects
    console.log(x);
    break;
  case 2:
    var x = 20; // Overwrites previous `x`
    console.log(x);
    break;
  default:
    console.log("Default case");
}

💡 Why not recommended?

• Variables declared with var remain accessible throughout the function.
• var does not respect block scope, which might lead to unexpected issues.

2. Declare the Variable Before the switch Block

You can declare the variable outside the switch and assign values inside the case blocks.

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let x;

switch (value) {
  case 1:
    x = 10;
    console.log(x);
    break;
  case 2:
    x = 20;
    console.log(x);
    break;
  default:
    x = 0;
    console.log("Default case", x);
}

💡 Why this works?

• The variable is only declared once.
• It avoids scope conflicts inside case blocks.

3. Use a Function to Encapsulate the Scope

You can use a function inside each case to ensure variable declarations don’t conflict.

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switch (value) {
  case 1:
    (function() {
      let x = 10;
      console.log(x);
    })();
    break;
  case 2:
    (function() {
      let x = 20;
      console.log(x);
    })();
    break;
  default:
    console.log("Default case");
}

💡 Why this works?

• Each case gets its own function scope, avoiding redeclaration issues.
• Helps keep code modular.

Best Approach?

• ✅ Use block { } for each case (clean and readable).
• ✅ Declare the variable outside if it needs to be shared.
• 🚫 Avoid var unless necessary.
• ✅ Use functions if encapsulation is needed.

Which approach fits your use case best? 🚀