ReferenceError in Temporal Dead Zone

console.log(foo); // ReferenceError: foo is not defined
let foo;

{
  console.log(foo); // ReferenceError: Cannot access 'foo' before initialization
  let foo;
}

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

The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created.

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

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment.

A LexicalEnvironment is a local lexical scope, e.g., for let-defined variables. If you define a variable with let in a catch block, it is only visible within the catch block, and to implement that in the spec, we use a LexicalEnvironment. VariableEnvironment is the scope for things like var-defined variables. vars can be thought of as "hoisting" to the top of the function.

• LexicalEnvironment(Local lexical Scope) : let 과 같은 변수가 선언될 때의 Scope

• VariableEnvironment : var 과 같은 변수가 선언될 때의 Scope

To implement this in the spec, we give functions a new VariableEnvironment, but say that blocks inherit the enclosing VariableEnvironment.

{ StatementList } 1. Let oldEnv be the running execution context's LexicalEnvironment. 2. Let blockEnv be NewDeclarativeEnvironment(oldEnv). 3. Perform BlockDeclarationInstantiation(StatementList, blockEnv). 4. Set the running execution context's LexicalEnvironment to blockEnv. 5. Let blockValue be the result of evaluating StatementList. 6. Set the running execution context's LexicalEnvironment to oldEnv. 7. Return blockValue.

console.log(foo); // ReferenceError: foo is not defined
let foo;

console.log(foo);

let foo;

{
  console.log(foo); // ReferenceError: Cannot access 'foo' before initialization
  let foo;
}