🔰initial value
🚧 施工中
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
const must be declared with an initial value❗
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.
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