🔰initial value

🚧 施工中

JS ⟩ variable ⟩ initial value

variable declaration can assign a value to a variable.

let i = 0, j = 0, k = 0;    // ⭐ in a single declaration
let [u, v] = [3, 4];        // 💡 destructuring assignment
let x = 2, y = x * x;       // ✅ use "previously declared"
let [p, q, ...rest] = [10, 20, 30, 40, 50];    // "rest" operator

// ❌ "destructuring assignment" can't use "previous element"
let [a, b] = [ 2, a * a ];  // ⛔ ReferenceError

⭐️ 重點

const must be declared with an initial value❗

if you don't specify an initial value for a variable, its value will be assigned undefined automatically.

• var hoisting - declaration hoisted, value initialized to "undefined".

• let/const/class hoisting - declaration hoisted, value "uninitialized"❗️

• const requires initialization - must have with initial value❗️

• function hoisting - declaration hoisted, value initialized to a function.

💈範例

• replit：initial value

let message = "hello";

// 💡 declare multiple variables
let i = 0, j = 0, k = 0;    // ⭐ in a single declaration
let [u, v] = [3, 4];        // ⭐ destructuring assignment

// 💡 destructuring assignment with "rest" operator
let [p, q, ...rest] = [10, 20, 30, 40, 50];

// ✅ initializers can use "previously declared variables"
let x = 2, y = x * x; 

// ❌ but no luck with "destructuring assignment"
// -----------------------------------------------------------
let [a, b] = [ 2, a * a ];  // ⛔ ReferenceError
//                ^
// ⛔ ReferenceError: Cannot access 'a' before initialization
// -----------------------------------------------------------

👉 ReferenceError： cannot access 'xxx' before initialization❗️

👥 相關

• hoistingwill / won't provide initial value for (var|function) / (let|const) declaration.

• default parameter uses initial values.

• uninitialized variable has no initial values, not undefined even❗
• destructuring assignment can be used to provide initial values.

📗 參考

• JavaScript: The Definitive Guide ⟩ 3.10 Variable Declaration & Assignment.