Stage 4 Draft / May 22, 2018

Optional catch binding

Introduction

Goals

1The try Statement

Syntax

TryStatement[Yield, Await, Return]:tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return] tryBlock[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return] tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return] Catch[Yield, Await, Return]:catch(CatchParameter[?Yield, ?Await])Block[?Yield, ?Await, ?Return] Finally[Yield, Await, Return]:finallyBlock[?Yield, ?Await, ?Return] CatchParameter[Yield, Await]:BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] TryStatement[Yield, Await, Return]:tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return] tryBlock[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return] tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return] Catch[Yield, Await, Return]:catch(CatchParameter[?Yield, ?Await])Block[?Yield, ?Await, ?Return] catchBlock[?Yield, ?Await, ?Return] Finally[Yield, Await, Return]:finallyBlock[?Yield, ?Await, ?Return] CatchParameter[Yield, Await]:BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] Note

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.

1.1Runtime Semantics: CatchClauseEvaluation

With parameter thrownValue.

Catch:catch(CatchParameter)Block
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let catchEnv be NewDeclarativeEnvironment(oldEnv).
  3. Let catchEnvRec be catchEnv's EnvironmentRecord.
  4. For each element argName of the BoundNames of CatchParameter, do
    1. Perform ! catchEnvRec.CreateMutableBinding(argName, false).
  5. Set the running execution context's LexicalEnvironment to catchEnv.
  6. Let status be the result of performing BindingInitialization for CatchParameter passing thrownValue and catchEnv as arguments.
  7. If status is an abrupt completion, then
    1. Set the running execution context's LexicalEnvironment to oldEnv.
    2. Return Completion(status).
  8. Let B be the result of evaluating Block.
  9. Set the running execution context's LexicalEnvironment to oldEnv.
  10. Return Completion(B).
Catch:catchBlock
  1. Return the result of evaluating Block.
Note

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

ACopyright & Software License

Copyright Notice

© 2018 Michael Ficarra

Software License

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