📜var

🚧 under construction

JS ⟩ variable ⟩ var

Stop using var❗ ( 👉see why❗)

( ⭐ var is a statement, not a declaration❗)

declares a variable in function scope / global scope ( and does a lot of side effects❗).

⭐️ 重點

var

• is a reserved word❗

• is a statement, not a declaration❗( 👉 var is a statement❗️❗)

• doesn't have block scope. ( 👉 var has no block scope❗️❗)

👉 compare： function declaration

JS is lenient with var, strict with let.

• var redeclaration applied even in strict mode❗️, even in strict mode❗

• let redeclaration not allowed even in sloppy mode❗️, even in sloppy mode❗

in global scope,

• var / function are implemented as global object property❗

• these properties cannot be deleted with delete❗

👉 global var / function is global object property❗️

Variable Statement

• var statement declares variables that are scoped to the running execution context's VariableEnvironment.

• var variables are created when their containing Environment Record is instantiated and are initialized to undefined when created.

• Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable.

• A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

📘 JS spec ⟩ Variable Statement

🔴 主題

• ✅stop using var❗️
• var is a statement❗️(side effects)

• global var / function is global object property❗️(side effect)

• accessing var before declaration gets undefined❗️(unexpected result)

• var has no block scope❗️ (unexpected result)

• var in block can't shadow outer let❗️(unexpected result)

• var can shadow parameter even in strict mode❗️(bad practice)

• var redeclaration applied even in strict mode❗️(bad practice)

🧨 雷區

• var has no block scope❗️❗
• accidental global variable in sloppy mode❗️❗
• var is a statement❗️❗

👥 相關

• var can shadow parameter even in strict mode❗️

• strict mode

📗 參考

• itHome ⟩ 你不可不知的 JavaScript 二三事 ⟩

  • #Day7：傳統 var 關鍵字的不足
  • #Day8：var 掰掰 — ES6 更嚴謹安全的 let 和 const

📘 手冊

• Statements and declarations ⟩ declaring variables ⟩ var
• JS spec ⟩

  • Variable Statement
  • VariableStatement

🗣 討論

• Is there any difference between a global variable and a property of the Global Object
• Where are arguments positioned in the lexical environment?

🚧

• VariableEnvironment