⛔no const for classic "for"

JS ⟩ statement ⟩ declaration ⟩ const ⟩ no const for classic for-loop

const

• can be used with for-in / for-of loop.

• can't be used with a classic for loop.

👉 no const for classic "for"❗

💈範例

• replit：classic for-loop can't use const

• 👉 ⛔️ TypeError ⟩ assignment to constant variable❗️

const { log } = console;

const array = [1,2,3];
const students = ['Joe', 'Jane', 'Jack'];

// ⭐️ for-in loop
//   ╭───╮ <------------------------- this is OK ✅
for (const index in students) {
    // this `const` behaves like:
    //
    //    // inside block scope
    //    const index = <next index in students>
    //
    log(`${index}: ${students[index]}`);
}

// ⭐️ for-of loop
//   ╭───╮ <------------------------- this is OK ✅
for (const value of array) {
    // this `const` behaves like:
    //
    //    // inside block scope
    //    const value = <next value of array>
    //
    log(value);
}

// ⭐️ classic for-loop❗
//
//   but this `const` behaves like: 
//
//       // in its own scope❗ (like function name scope)
//       // (inside for-loop but outside block scope)
//       // (between outer scope and inner block scope)
//
//       const i = 0;        // ⭐️ reassignment will fail❗
//
//  ╭─ for-loop init scope ─╮
//   ╭───╮ <------------------------- this is NOT OK ❌
for (const i = 0; i < 3; i++) {
    //                   ^^^
    // TypeError after the first iteration❗
    // (⛔ TypeError: Assignment to constant variable)
    log(i);
}

// output
// ---------------------
// 0: Joe
// 1: Jane
// 2: Jack
// 1
// 2
// 3
// 0 <----- ⭐️ first iteration (then fails)
// ⛔ TypeError: Assignment to constant variable.

📗 參考