spec.html

<pre class="metadata">
title: Symbol.thenable
status: proposal
stage: 0
location: https://gus.host/proposal-import-meta/
copyright: false
contributors: Gus Caplan
</pre>
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<emu-intro id=sec-intro>
  <h1>Introduction</h1>
  <p>This proposal introduces the well-known symbol @@thenable, which is used to determine if an object passed through Promise.resolve should be treated as a thenable.</p>
</emu-intro>

<emu-clause id="sec-ecmascript-language-types-symbol-type">
  <h1>The Symbol Type</h1>
  <!-- es6num="6.1.5.1" -->
  <emu-clause id="sec-well-known-symbols">
    <h1>Well-Known Symbols</h1>
    <p>Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (<emu-xref href="#sec-code-realms"></emu-xref>).</p>
    <p>Within this specification a well-known symbol is referred to by using a notation of the form @@name, where &ldquo;name&rdquo; is one of the values listed in <emu-xref href="#table-1"></emu-xref>.</p>
    <emu-table id="table-1" caption="Well-known Symbols">
      <table>
        <tbody>
        <tr>
          <th>
            Specification Name
          </th>
          <th>
            [[Description]]
          </th>
          <th>
            Value and Purpose
          </th>
        </tr>
        <tr>
          <td>
            @@asyncIterator
          </td>
          <td>
            `"Symbol.asyncIterator"`
          </td>
          <td>
            A method that returns the default AsyncIterator for an object. Called by the semantics of the `for`-`await`-`of` statement.
          </td>
        </tr>
        <tr>
          <td>
            @@hasInstance
          </td>
          <td>
            `"Symbol.hasInstance"`
          </td>
          <td>
            A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the `instanceof` operator.
          </td>
        </tr>
        <tr>
          <td>
            @@isConcatSpreadable
          </td>
          <td>
            `"Symbol.isConcatSpreadable"`
          </td>
          <td>
            A Boolean valued property that if true indicates that an object should be flattened to its array elements by <emu-xref href="#sec-array.prototype.concat">`Array.prototype.concat`</emu-xref>.
          </td>
        </tr>
        <tr>
          <td>
            @@iterator
          </td>
          <td>
            `"Symbol.iterator"`
          </td>
          <td>
            A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
          </td>
        </tr>
        <tr>
          <td>
            @@match
          </td>
          <td>
            `"Symbol.match"`
          </td>
          <td>
            A regular expression method that matches the regular expression against a string. Called by the <emu-xref href="#sec-string.prototype.match">`String.prototype.match`</emu-xref> method.
          </td>
        </tr>
        <tr>
          <td>
            @@replace
          </td>
          <td>
            `"Symbol.replace"`
          </td>
          <td>
            A regular expression method that replaces matched substrings of a string. Called by the <emu-xref href="#sec-string.prototype.replace">`String.prototype.replace`</emu-xref> method.
          </td>
        </tr>
        <tr>
          <td>
            @@search
          </td>
          <td>
            `"Symbol.search"`
          </td>
          <td>
            A regular expression method that returns the index within a string that matches the regular expression. Called by the <emu-xref href="#sec-string.prototype.search">`String.prototype.search`</emu-xref> method.
          </td>
        </tr>
        <tr>
          <td>
            <ins>@@thenable</ins>
          </td>
          <td>
            <ins>`"Symbol.thenable"`</ins>
          </td>
          <td>
            <ins>A boolean valued property whose value controls whether the associated object will be considered "thenable." Accessed by <emu-xref href="#sec-promise-resolve-functions">Promise Resolve Functions</emu-xref>.</ins>
          </td>
        </tr>
        <tr>
          <td>
            @@species
          </td>
          <td>
            `"Symbol.species"`
          </td>
          <td>
            A function valued property that is the constructor function that is used to create derived objects.
          </td>
        </tr>
        <tr>
          <td>
            @@split
          </td>
          <td>
            `"Symbol.split"`
          </td>
          <td>
            A regular expression method that splits a string at the indices that match the regular expression. Called by the <emu-xref href="#sec-string.prototype.split">`String.prototype.split`</emu-xref> method.
          </td>
        </tr>
        <tr>
          <td>
            @@toPrimitive
          </td>
          <td>
            `"Symbol.toPrimitive"`
          </td>
          <td>
            A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
          </td>
        </tr>
        <tr>
          <td>
            @@toStringTag
          </td>
          <td>
            `"Symbol.toStringTag"`
          </td>
          <td>
            A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method <emu-xref href="#sec-object.prototype.tostring">`Object.prototype.toString`</emu-xref>.
          </td>
        </tr>
        <tr>
          <td>
            @@unscopables
          </td>
          <td>
            `"Symbol.unscopables"`
          </td>
          <td>
            An object valued property whose own and inherited property names are property names that are excluded from the `with` environment bindings of the associated object.
          </td>
        </tr>
        </tbody>
      </table>
    </emu-table>
  </emu-clause>
</emu-clause>

<!-- es6num="25.4.1" -->
<emu-clause id="sec-promise-abstract-operations">
  <h1>Promise Abstract Operations</h1>

  <!-- es6num="25.4.1.3.2" -->
  <emu-clause id="sec-promise-resolve-functions">
    <h1>Promise Resolve Functions</h1>
    <p>A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.</p>
    <p>When a promise resolve function _F_ is called with argument _resolution_, the following steps are taken:</p>
    <emu-alg>
      1. Assert: _F_ has a [[Promise]] internal slot whose value is an Object.
      1. Let _promise_ be _F_.[[Promise]].
      1. Let _alreadyResolved_ be _F_.[[AlreadyResolved]].
      1. If _alreadyResolved_.[[Value]] is *true*, return *undefined*.
      1. Set _alreadyResolved_.[[Value]] to *true*.
      1. If SameValue(_resolution_, _promise_) is *true*, then
        1. Let _selfResolutionError_ be a newly created *TypeError* object.
        1. Return RejectPromise(_promise_, _selfResolutionError_).
      1. If Type(_resolution_) is not Object, then
        1. Return FulfillPromise(_promise_, _resolution_).
      1. <ins>Let _thenable_ be Get(_resolution_, @@thenable)</ins>
      1. <ins>If _thenable_ is *false*, then</ins>
        1. <ins>Return FulfillPromise(_promise_, _resolution_).</ins>
      1. Let _then_ be Get(_resolution_, `"then"`).
      1. If _then_ is an abrupt completion, then
        1. Return RejectPromise(_promise_, _then_.[[Value]]).
      1. Let _thenAction_ be _then_.[[Value]].
      1. If IsCallable(_thenAction_) is *false*, then
        1. Return FulfillPromise(_promise_, _resolution_).
      1. Perform EnqueueJob(`"PromiseJobs"`, PromiseResolveThenableJob, &laquo; _promise_, _resolution_, _thenAction_ &raquo;).
      1. Return *undefined*.
    </emu-alg>
    <p>The `length` property of a promise resolve function is 1.</p>
  </emu-clause>
</emu-clause>


<emu-clause id="sec-reflection">
  <h1>Reflection</h1>
  <!-- es6num="26.3" -->
  <emu-clause id="sec-module-namespace-objects">
    <h1>Module Namespace Objects</h1>
    <p>A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an |ImportDeclaration| that includes a |NameSpaceImport|.</p>
    <p>In addition to the properties specified in <emu-xref href="#sec-module-namespace-exotic-objects"></emu-xref> each Module Namespace Object has the following own <del>property</del><ins>properties</ins>:</p>

    <!-- es6num="26.3.1" -->
    <emu-clause id="sec-@@tostringtag">
      <h1>@@toStringTag</h1>
      <p>The initial value of the @@toStringTag property is the String value `"Module"`.</p>
      <p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</p>
    </emu-clause>

    <emu-clause id="sec-@@thenable">
      <h1><ins>@@thenable</ins></h1>
      <p><ins>The initial value of the @@thenable property is the Boolean value *false*.</ins></p>
      <p><ins>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</ins></p>
    </emu-clause>
  </emu-clause>
</emu-clause>