❗var is a statement❗️

🚧 under construction

JS ⟩ variable ⟩ var ⟩ var is statement❗

(side effects)

var is a statement, instead of a declaration：

• it doesn't follow normal lexical scoping rules.

• may create side effects：

  • global var / function is global object property❗️

  • var redeclaration applied even in strict mode❗️

  • var has no block scope❗️

⭐️ 重點

📘 JS spec ⟩ Variable Statement

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

• var variable(s) 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.

👥 相關

📘 手冊

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