🔰variable

JS ⟩ variable

a variable has a valueand an identifier as its name.

🈯 synonyms： "binding"

🧨 雷區

assignment to an undeclared identifier will result in an accidental global variable in sloppy mode❗️ in sloppy mode.

⭐️ 重點

A "variable" is just a property of the environment record (associated with the currently executing block /function /module /script).

variables just been declared will be assigned undefined automatically (with some nuances).

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

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

• const requires initializationinitial value

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

(other than declaration) all occurrences of variables/identifiers serve in one of two "roles"：

• the target of an assignment. (LHS reference

• the source of a value. (RHS reference)

var redeclaration applied even in strict mode❗️ (even in strict mode)❗

a closure (or IIFE) can be used to create a scope to hide variables/functions.

// ⭐️ states of an identifier/variable (in a specific scope)
//    • undeclared identifier
//   -> uninitialized variable (declared but undefined) 
//   -> initialized variable (with value !== undefined)

🔴 主題

• declaring variables

  • variable declaration / variable redeclaration

  • var / let / const

  • parameter - parameters are variables.

• referencing variables

  • global variable

  • RHS reference / LHS reference

  • undeclared identifierundeclared

  • temporal dead zone of a let/const variable.

  • uninitialized variable

• variable hoisting

  • let/const/class hoisting

  • var hoisting

  • function expression not hoisted

  • compare：function hoisting

• other topics

  • variable shadowing

  • can't delete variable/function❗️

🗺️ 圖解

a variable

👥 相關

• let declares a block scoped local variable.

📗 參考

• 3 Ways to Check if a Variable is Defined in JavaScript

• JavaScriptTutorial ⟩ JavaScript Variables

• Eloquent JavaScript ⟩ Binding

• YDKJS: Scope & Closures (v.2) ⟩ Ch. 1 ⟩ Compiler Speak

• JS.info ⟩ Variables

📘 手冊

• Variable