Stage 0 Draft / August 1, 2025

ECMAScript Try Operator

Introduction

Note

Important Notice: Until this proposal is accepted by ECMA, this spec.emu file might not reflect the latest updates to the proposal. Readers should base their understanding primarily on the current README.md file, which serves as the authoritative source during the proposal stage. This specification is an imagination of what the final specification might look like and while it can be trusted as a reference, the README.md has higher importance at this stage of the project.

This proposal introduces a try operator and Result class to JavaScript for improved error handling ergonomics. The try operator evaluates an expression within an implicit try-catch block and returns a Result instance containing either the successful value or the caught error.

1 Assignment Operators

Syntax

AssignmentExpression[In, Yield, Await] : TryExpression[?In, ?Yield, ?Await] TryExpression[In, Yield, Await] : try [no LineTerminator here] [lookahead ≠ {] AssignmentExpression[?In, ?Yield, ?Await] Note

Placing the try operator in assignment expression allows it to protect an entire assignment expression.

1.1 Runtime Semantics: Evaluation

TryExpression : try AssignmentExpression
  1. Let A be Completion(Evaluation of Expression).
  2. If A is an abrupt completion, return ? TryExpressionResult(A).
  3. Let B be Completion(GetValue(A)).
  4. Return ? TryExpressionResult(B).
 Note 1

GetValue must be called even though its value is not used because it may have observable side-effects.

 Note 2

This is identical to evaluation of a try statement.

1.2 TryExpressionResult ( result )

The abstract operation TryExpressionResult takes argument result (a Completion Record) and returns either a normal completion containing a Result or an abrupt completion. It performs the following steps when called:

  1. If result is a normal completion, return Result.ok(result.[[VALUE]]).
  2. If result is a throw completion, return Result.error(result.[[VALUE]]).
  3. Return ? result.

1.3 Result.ok ( value )

The abstract operation Result.ok takes argument value (an ECMAScript language value) and returns a Result. It is the abstract equivalent of calling Result.ok(value)

1.4 Result.error ( value )

The abstract operation Result.error takes argument value (an ECMAScript language value) and returns a Result. It is the abstract equivalent of calling Result.error(value)

2 The Result Constructor

class Result {
  constructor(ok, error, value) {
    ok = Boolean(ok)
    this.ok = ok
    if (ok) {
      this.value = value
    } else {
      this.error = error
    }
  }
  *[Symbol.iterator]() {
    yield this.ok
    yield this.error
    yield this.value
  }
  static ok(value) {
    return new Result(true, undefined, value)
  }
  static error(error) {
    return new Result(false, error, undefined)
  }
  /* a convenience method for user-land, not used by the try operator */
  static try(arg, ...args) {
    try {
      let result;
      if (/* IsCallable(arg) */typeof arg === "function") {
        result = arg.apply(undefined, args)
      } else {
        result = arg;
      }
      if (/* IsPromise (result) */result instanceof Promise) {
        return result.then(Result.ok, Result.error)
      } else {
        return Result.ok(result);
      }
    } catch (e) {
      return Result.error(e)
    }
  }
}

A Copyright & Software License

Copyright Notice

© 2025 Arthur Fiorette, Arlen Beiler

Software License

All Software contained in this document ("Software") is protected by copyright and is being made available under the "BSD License", included below. This Software may be subject to third party rights (rights from parties other than Ecma International), including patent rights, and no licenses under such third party rights are granted under this license even if the third party concerned is a member of Ecma International. SEE THE ECMA CODE OF CONDUCT IN PATENT MATTERS AVAILABLE AT https://ecma-international.org/memento/codeofconduct.htm FOR INFORMATION REGARDING THE LICENSING OF PATENT CLAIMS THAT ARE REQUIRED TO IMPLEMENT ECMA INTERNATIONAL STANDARDS.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  3. Neither the name of the authors nor Ecma International may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE ECMA INTERNATIONAL "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL ECMA INTERNATIONAL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.