ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

14.2 Block

Syntax

BlockStatement[Yield, Await, Return] : Block[?Yield, ?Await, ?Return] Block[Yield, Await, Return] : { StatementList[?Yield, ?Await, ?Return]opt } StatementList[Yield, Await, Return] : StatementListItem[?Yield, ?Await, ?Return] StatementList[?Yield, ?Await, ?Return] StatementListItem[?Yield, ?Await, ?Return] StatementListItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] Declaration[?Yield, ?Await]

14.2.1 Static Semantics: Early Errors

Block : { StatementList }

14.2.2 Runtime Semantics: Evaluation

Block : { }
  1. Return empty.
 Block : { 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 Completion(Evaluation of StatementList).
  6. Set the running execution context's LexicalEnvironment to oldEnv.
  7. Return ? blockValue.
 Note 1

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

 StatementList : StatementList StatementListItem
  1. Let sl be ? Evaluation of StatementList.
  2. Let s be Completion(Evaluation of StatementListItem).
  3. Return ? UpdateEmpty(s, sl).
 Note 2

The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

14.2.3 BlockDeclarationInstantiation ( code, env )

The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node) and env (a Declarative Environment Record) and returns unused. code is the Parse Node corresponding to the body of the block. env is the Environment Record in which bindings are to be created.

Note

When a Block or CaseBlock is evaluated a new Declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

It performs the following steps when called:

  1. Let declarations be the LexicallyScopedDeclarations of code.
  2. Let privateEnv be the running execution context's PrivateEnvironment.
  3. For each element d of declarations, do
    1. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ! env.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ! env.CreateMutableBinding(dn, false). NOTE: This step is replaced in section B.3.2.6.
    2. If d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
      1. Let fn be the sole element of the BoundNames of d.
      2. Let fo be InstantiateFunctionObject of d with arguments env and privateEnv.
      3. Perform ! env.InitializeBinding(fn, fo). NOTE: This step is replaced in section B.3.2.6.
  4. Return unused.