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 errors by type.

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 will be able to 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

This proposal introduces a new Binding type for the Catch statement called BindingGuard; these are some of the options for this new type.

Option 1 (using `as`):

class ConflictError extends Error {}
class NotFoundError extends Error {}
class OtherError extends Error {}
  
try {
  something();
} catch (ConflictError as conflict) {
  // handle it one way...
} catch(NotFoundError || OtherError as other) {
  // handle the other way...
} catch (err) {
  // catch all...
}

Option 2 (using `if instanceOf`):

class ConflictError extends Error {}
class NotFoundError extends Error {}
class OtherError extends Error {}
  
try {
  something();
} catch (conflict if instanceOf ConflictError) {
  // handle it one way...
} catch(other if instanceOf NotFoundError || instanceOf OtherError) {
  // handle the other way...
} catch (err) {
  // catch all...
}

Option 3 (using `instanceOf`):

class ConflictError extends Error {}
class NotFoundError extends Error {}
class OtherError extends Error {}
  
try {
  something();
} catch (conflict instanceOf ConflictError) {
  // handle it one way...
} catch(other instanceOf NotFoundError || instanceOf OtherError) {
  // handle the other way...
} catch (err) {
  // catch all...
}

Option 4 (using `:`):

Possible conflict with Types as comment proposal

class ConflictError extends Error {}
class NotFoundError extends Error {}
class OtherError extends Error {}
  
try {
  something();
} catch (conflict: ConflictError) {
  // handle it one way...
} catch(other: NotFoundError | OtherError) {
  // handle the other way...
} catch (err) {
  // catch all...
}

Implementations

Babel Plugin //TBD

Q&A

What if the error is a string?

We are trying to solve for types only, so this would work:

try {
  something();
} catch (String as err) {
  // handle it...
}

But it would catch all errors thrown as strings.

michaelficarra/proposal-catch-guards