❗function in block (FiB)

a "function declaration" defined in a "block".

JS ⟩ declaration ⟩ function ⟩ function in block (FiB)

Don't use function (declaration) in block❗

a function declaration defined in a block.

• sloppy mode: hoisted / initialized to undefined (outside block)

• strict mode: local to the block.

⭐️ 重點

JS spec says that "function declaration in block is block-scoped" ❗

However, in most browser-based JS engines (including v8)：

• the identifier of the FiB is scoped outside the block

• with its value initialized to undefined.

📗 You Don't Know JS

function (declaration) in block

• in sloppy mode：

  • is hoisted and visible outside the block❗

  • can be considered as a redeclaration

    • if there's already a function with the same name in the outer scope

    • the (re)assignment is not applied until the block is run❗

• in strict mode：

  • is local to the block❗

  • never is a redeclaration any more❗

👉 compare： var

recommendations for function in block (FiB)

• don't use it❗

💈範例

• replit：pure FiB

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

// undeclared;    // ⛔ ReferenceError: 'undeclared' is not defined

// ❗ access before declaration
console.log(pureFiB === undefined);    
// • sloppy: ✅ true
// • strict: ⛔ ReferenceError: 'pureFiB' is not defined

// block
{
    // ⭐️ "pure" function in block
    function pureFiB() {
        console.log(`I'm "Pure FiB".`);
    }
}

// ❗ "function in block" has no "block scope"
// -----------------------------------------------
// • sloppy: ✅ hoisted / initialized to undefined (in outer scope)
// • strict: ⛔ ReferenceError (invisible in outer scope)
pureFiB();    
// • sloppy: ✅ I'm "Pure FiB".
// • strict: ⛔ ReferenceError: 'pureFiB' is not defined

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩ Function Declarations in Blocks (FiB) ⭐️

🗣 討論

• Function Declarations Within Blocks according to the Google JavaScript style guide