Support optional try/catch binding as added in Node 10 & V8 6.6

[Avuxo]

Support for this ECMAScript Stage 4 proposal. This has already been implemented in v8 6.6 and as a result is already implemented in new versions of Google Chrome/Chromium and Node.js 10. This language feature is used in the Node.js `assert' standard library, so any users who attempt to apply the parser to that file will encounter this error.

Previously, try/catch statements required a binding to be syntactically valid. However, with this change they are no longer a requirement as the binding is now optional.

try {
    // something exceptional
} catch {
    // no longer has a required parameter
}

This issue would be a fairly simple implementation and would likely not add any significant overhead to the parsing process.

Steps to reproduce

// run in any environment using V8 6.6: Google Chrome or Node 10.x
esprima.parse('try { new Error(); } catch {}');

Expected output

// AST
{
  "type": "Program",
  "body": [
    {
      "type": "TryStatement",
      "block": {
        "type": "BlockStatement",
        "body": [
          {
            "type": "ExpressionStatement",
            "expression": {
              "type": "NewExpression",
              "callee": {
                "type": "Identifier",
                "name": "Error"
              },
              "arguments": []
            }
          }
        ]
      },
      "handler": {
        "type": "CatchClause",
        "param": null,
        "body": {
          "type": "BlockStatement",
          "body": []
        }
      },
      "finalizer": null
    }
  ],
  "sourceType": "script"
}

Actual output

/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:1995
	        throw this.unexpectedTokenError(token, message);
	        ^
Error: Line 1: Unexpected token {
    at ErrorHandler.constructError (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:5012:22)
    at ErrorHandler.createError (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:5028:27)
    at Parser.unexpectedTokenError (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:1985:39)
    at Parser.throwUnexpectedToken (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:1995:21)
    at Parser.expect (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:2168:19)
    at Parser.parseCatchClause (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:4049:15)
    at Parser.parseTryStatement (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:4080:58)
    at Parser.parseStatement (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:4157:43)
    at Parser.parseStatementListItem (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:3388:39)
    at Parser.parseScript (/Users/ben/workspace/testing/catch-binding/node_modules/esprima/dist/esprima.js:4723:29)

Relevant references

• Stage 4 ECMAScript TC39 proposal
• v8 6.6 change log

[ariya]

Hi @Avuxo, thanks for the report.

For a new language feature, we usually need more details, see this guide. For some examples, see e.g. #1550. Please review the issue description to include the necessary information.

[JosephPecoraro]

This syntax has been supported in browsers for nearly a year! Lets see what we can do to move this forward.

[JosephPecoraro]

ES2019 Feature: Optional catch binding: Tracking Issue

Syntax:

try { } catch { }

New production for Catch that just doesn't require a binding:

 Catch[Yield, Await, Return]:
     catch ( CatchParameter[?Yield, ?Await] ) Block[?Yield, ?Await, ?Return]
+    catch Block[?Yield, ?Await, ?Return]

Spec:

https://github.com/tc39/proposal-optional-catch-binding
https://tc39.es/proposal-optional-catch-binding/

Remaining Tasks:

• Update Issue 1953 catch clause #1954

Additional considerations

• Supported and shipped in all major Browsers: Safari, Chrome, Firefox
• Supported in JavaScript environments: jsc, node
• Proposed AST change is that CatchClause will now have an optional (null) param

[JosephPecoraro]

This was added to ESTree for ES2019:
https://github.com/estree/estree/blob/master/es2019.md

---

This document specifies the extensions to the core ESTree AST types to support the ES2019 grammar.

Statements

CatchClause

extend interface CatchClause {
    param: Pattern | null;
}

The param is null if the catch binding is omitted. E.g., try { foo() } catch { bar() }

[ariya]

Thanks for adding some more important details @JosephPecoraro!

[darrnshn]

Any update on this? It still seems to be invalid on the demo page: https://esprima.org/demo/parse.html?code=try%20%7B%7D%20catch%20%7B%7D

We would really appreciate this fix! Thanks!