🔰var hoisting

identifier hoisted to top of "function scope", value initialized to `undefined`❗️

JS ⟩ scope ⟩ hoisting ⟩ var hoisting

with a var declaration

• identifier： hoisted to the top of its enclosing function scope. ( 👉 var has no block scope❗️❗)

• value： initialized to undefined❗

✨ 範例

• function expression not hoisted❗

💈範例

• replit：var hoisting

• 👉 ⛔️ TypeError： 'xxx' is not a function❗️

const { log } = console;

// ⭐️ var hoisting
// ----------------
// • identifier: IS hoisted❗
// • value     : initialized to `undefined`❗

// <-------- greeting  === undefined
// <-------- greeting2 === undefined

// ⭐️ identifiers `greeting`, `greeting2`:
// • can be referenced now❗ (no ReferenceError)
log(greeting,  typeof greeting );  // undefined, 'undefined'
log(greeting2, typeof greeting2);  // undefined, 'undefined'

// ⭐️ reassign before declaration is OK 
greeting2 = "Hello!";            
log(greeting2);                    // Hello!

// ⭐️ identifier `greeting`:
// • can't be called (not a "function") yet❗(⛔ TypeError)
greeting();    // ⛔ TypeError: 'greeting' is not a function

// ⭐️ the following are "var declarations"
// ⭐️ "var declaration" is related to "var hoisting"
// ------------------------------------------------------
// • identifier: hoisted to top of scope❗
// • value     : initialized to `undefined`❗
// ------------------------------------------------------

// ⭐️ var declarations

//  ╭─ id ─╮   ╭── value ──╮  (value: function expression)
var greeting = function() {
    console.log("Hello!");
};

//  ╭─ id ──╮   ╭─value─╮     (value: string)
var greeting2 = "Howdy!" ;

👥 相關

• compare：function hoisting / let/const/class hoisting

• var redeclaration applied even in strict mode❗️❗

• var has no block scope❗️❗ - var only has function scope.

• function expression not hoisted

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩

  • Ch. 1 ⟩ Hoisting

  • Ch. 5 ⟩ Hoisting: Declaration vs. Expression