⛔cannot access 'xxx' before initialization❗️

JS ⟩ error ⟩ ReferenceError ⟩ cannot access '...' before initialization

⛔ ReferenceError：cannot access '...' before initialization❗

replit：TDZ: let/const/class

using typeof on lexical declaration (let/ const/ class) in its temporal dead zone will throw a ReferenceError.

// ⭐ temporal dead zone (start)
// ------------------------------
typeof aLet;       // ⛔ ReferenceError: Cannot access 'aLet' before initialization
typeof aConst;     // ⛔ ReferenceError: Cannot access 'aConst' before initialization
typeof aClass;     // ⛔ ReferenceError: Cannot access 'aClass' before initialization

typeof undeclared;        // ❗ 'undefined'

// ⭐ lexical declarations (let/const/class)
// --------------------------------------------
let aLet;                 // ⭐ initialized to `undefined`
const aConst = "hello";
class aClass {}

replit：hoisting & TDZ

referring to a variable before initialization causes a runtime error.

const { log } = console;

function sayHi() {
    
    var greeting = "Hello";

    // block scope
    {
        
    // --------------------------
    // ⭐️ cause runtime error❗
    // --------------------------
        greeting = "Howdy";                              // ╮
    //  ^^^^^^^^                                         // ⭐️
    // ⛔ ReferenceError: (runtime error)                // Temporal
    //    Cannot access 'greeting'                       // Dead
    //    before initialization                          // Zone (TDZ)
                                                         // for `greeting`
                                                         // ╯
        
        let greeting = "Hi";   // ⭐️ `greeting` declared here❗
        
        log(greeting);
    }
    
}

log('hello');    // ✅ 'hello' (this line is OK)
sayHi();         // ⛔ run-time error

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Last updated 3 years ago