❗redeclaration

🚧 施工中

JS ⟩ grammar ⟩ declaration ⟩ redeclaration

🚧

⭐️ 重點

JS is lenient with var, strict with let.

• var redeclaration applied even in strict mode❗️, even in strict mode❗

• let redeclaration not allowed even in sloppy mode❗️, even in sloppy mode❗

"function redeclaration" behaves like "var redeclaration".

• the declaration part is ignored.

• the assignment part is applied❗( even in strict mode❗)

👉 var redeclaration applied even in strict mode❗️ / 💈範例

🔴 主題

• variable redeclaration

  • var redeclaration

  • let redeclaration

• function redeclaration

  • function in block (FiB) - treated as "redeclaration" in sloppy mode.

👥 相關

• variable declaration

• variable shadowing

💈範例

• replit：re-declaration

// 'use strict';    // ⭐ toggle sloppy/strict mode

// ⭐ "redeclarations": (2)
// ------------------------------
// • assignments are always applied. 
//   (in both sloppy/strict mode)

// ⭐ "function in block": (3)(4)
// ------------------------------
// • sloppy mode:
//   • is visible outside the block.
//   • can be considered a "redeclaration".
//   • but the assignment is not applied until the block is run❗
//     ((3) is NOT run, (4) is run.)
//
// • strict mode:
//   • is "local" to the block.
//   • never is a "redeclaration".

// ⭐ "if-false-block"
if (false) {
    // (3)
    // -------------------------------
    // • sloppy mode:
    //   "redeclaration" NEVER happens. (because it never runs)
    // • strict mode:
    //   "function in block" is local to the block,
    //   it never is a "redeclaration".
    function foo() { return 3 }            // this line never runs❗
}

// ⭐ "if-true-block"
if (true) {
    // (4)
    // ---------------------------------
    // • sloppy mode:
    //   "redeclaration" DOES happen. (because it runs)
    // • strict mode:
    //   "function in block" is local to the block,
    //   it never is a "redeclaration".
    function foo() { return 4 }            // this line always runs❗
}

// (1)
// ⭐️ function declaration
// -------------------------
function foo() { return 1 }

// (2)
// ❗ function re-declaration
// ---------------------------
// • declaration: ignored
// • assignment : applied❗
function foo() { return 2 }

// log
console.log(foo());        // • sloppy: 4 ❗
                           // • strict: 2 ❗

📘 手冊

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