ECMAScript Catch Guards

This proposal adds the catch guards to the language, enabling developers to catch exceptions only when they match a specific class or set of classes.

This proposal draws heavily from similar features available in Java, C#, Ruby, and Python.

Problem statement

Javascript doesn't provide an ergonomic way of treating different types of errors.

Status

Champion: Willian Martins (Netflix, @wmsbill),

Stage: 0

Lastest update: draft created

Motivation

• Developer ergonomics:

  Today, developers have to catch all Errors, add an if or switch statement and rethrow even when they want to catch one particular type of error:

  class ConflictError extends Error {}
  
  try {
    something();
  } catch (err) {
    if (err instanceOf ConflictError) {
      // handle it...
    }
    throw err;
  }

• Parity with other languages:

  Scoping error handling to specific types is a common construct in several programming languges, as: Java, C#, Ruby, and Python.

• Code/Engine Optimization:

Engines can skip the block entirely if the error doesn't match the correct type.

Syntax options

The current implementation uses the following syntax:

 catch ( CatchParameter[?Yield, ?Await] ) { block }

Where CatchParameter is:

• BindingIdentifier[?Yield, ?Await]
• BindingPattern

Option 1 (New Catch Statement):

This proposal introduces a new Binding type for the CatchParameter called BindingGuard; this new bind type's shape and semantics are subject to stage 1 development.

  
try {
  something();
} catch (BindingGuard) {
  // handle it one way...
} catch(BindingGuard) {
  // handle the other way...
} catch (err) {
  // catch all...
}

Option 2 (New CatchPatten statement):

This option can be used in conjunction with option one or leveraged on the Pattern Matching proposal to bind different error patterns (syntax and semantics are subject to stage 1 development), making the catch statement keep its current syntax.

  
try {
  something();
} CatchPatten (BindingGuard) {
  // handle it one way...
} CatchPatten (BindingGuard) {
  // handle the other way...
} catch (err) {
  // catch all...
}

Implementations

• Babel Plugin //TBD