🔰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

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.

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❗️

Previouslet can't shadow parameter even in sloppy mode❗️Nextvariable redeclaration

Last updated 2 years ago

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

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 // -----------------------------------------------------------