TC-39 Proposal for additional is{Type} APIs
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There are a number of situations where existing type checking using instanceof can be problematic. For instance:

$ ./node
> (new Date()) instanceof Date
> (vm.runInNewContext('new Date()')) instanceof Date

In this case, both statements return valid Date objects. However, because the second is created in a separate realm, it is not recognized as a Date in the current realm, despite operating appropriately in every other respect.

In other cases, instanceof does not provide adequate granularity, such as checking if a given argument is an unsigned 16-bit integer vs. a signed 32-bit integer.

This proposal introduces a new Builtin built-in object that exposes methods that allow reliable cross-realm type checking for ECMAScript built-ins.

Prior art

Node.js has relied on such checks, in part, to reliably determine types for debugging, inspection and display formatting purposes in the util.format() and util.inspect() APIs. In addition, the is package on npm (which implements similar type checks) currently has roughly 33k+ downloads per day.

Node.js can (and has) implement these functions in a host-specific manner as part of the Node.js API but the preference would be towards having these kind of type checks be a regular part of the language API.

For example:

$ ./node
> util.isDate(new Date())
> util.isDate(vm.runInNewContext('new Date()'))
> vm.runInNewContext('new Date()') instanceof Date


What is needed?

  • Mechanism for reliably determining if any given object is a built-in or is an instance of a built-in, even across realms.
  • Mechanism for reliably determining if objects from different realms correspond to the same built-in (e.g. Date from one realm is the same built-in as Date from a second realm).
  • Avoid introducing new, or changing existing, language syntax.
  • Allow host environments to insert new built-ins.
  • Allow user code objects to masquerade as built-ins.

Proposed API

Identifying an Object as a Built-in

An object is identified as a built-in using:

  • A new [[Builtin]] internal slot to mark built-ins
  • A new @@builtin symbol (Symbol.builtin) property whose value is a function whose default behavior is to provide the value of the [[Builtin]] internal slot.

[[Builtin]] internal slot

Intrinsic objects listed in the table below have a [[Builtin]] internal slot with the given string value. Intrinsic objects not listed in the table do not have the [[Builtin]] internal slot.

Intrinsic Name Builtin Name
%Array% 'Array'
%ArrayBuffer% 'ArrayBuffer'
%AsyncFunction% 'AsyncFunction'
%Atomics% 'Atomics'
%Boolean% 'Boolean'
%DataView% 'DataView'
%Date% 'Date'
%Error% 'Error'
%EvalError% 'EvalError'
%Float32Array% 'Float32Array'
%Float64Array% 'Float64Array'
%Function% 'function'
%GeneratorFunction% 'GeneratorFunction'
%Int8Array% 'Int8Array'
%Int16Array% 'Int16Array'
%Int32Array% 'Int32Array'
%Map% 'Map'
%Math% 'Math'
%Number% 'Number'
%Object% 'object'
%Promise% 'Promise'
%Proxy% 'Proxy'
%RangeError% 'RangeError'
%ReferenceError% 'ReferenceError'
%Reflect% 'Reflect'
%RegExp% 'RegExp'
%Set% 'Set'
%SharedArrayBuffer% 'SharedArrayBuffer'
%String% 'String'
%Symbol% 'symbol'
%SyntaxError% 'SyntaxError'
%TypeError% 'TypeError'
%Uint8Array% 'Uint8Array'
%Uint8ClampedArray% 'Uint8ClampedArray'
%Uint16Array% 'Uint16Array'
%Uint32Array% 'Uint32Array'
%URIError% 'URIError'
%WeakMap% 'WeakMap'
%WeakSet% 'WeakSet'

Note: Currently, intrinsic prototype objects such as %DatePrototype% intentionally do not have a [[Builtin]] internal slot. The effect of this is such that Builtin.typeOf(new Date()) would return 'Date', Builtin.typeOf(Object.getPrototypeOf(new Date())) would return 'object', despite %DatePrototype% being an intrinsic object. The justification for this is that it is not yet clear if intrinsic prototype objects need to be identifiable as built-ins.

In addition, all built-in non-constructor functions and methods have a [[Builtin]] internal slot equal to the name of the function. These are used to allow using to determine if two function/method instances represent the same intrinsic function or method.

For instance,, vm.runInNewContext('eval'));                // true,
           vm.runInNewContext('Object.prototype.toString')); // true


The initial value of the @@builtin own property for all intrinsic objects having a [[Builtin]] internal slot is the same function that returns the value of the [[Builtin]] internal slot. Intrinsic objects that do not have the [[Builtin]] internal slot do not have an initial value for the @@builtin own property.

const builtIn1 = Date[Symbol.builtin];
const builtIn2 = Uint8Array[Symbol.builtin];
const same = builtIn1 === builtIn2;         // true

An object is detectable as a built-in if it has the @@builtin own property.

An object is detectable as an instance of a built-in if its constructor has a @@builtin property as either an own or inherited property.

class Foo {
  static [Symbol.builtin]() {
    return 'Foo';
class Bar extends Foo {}

Builtin.typeOf(new Foo());     // 'Foo'

Builtin.typeOf(new Bar());     // 'Foo'

Setting the @@builtin property to a non-function value makes the object, or instances of the object, no longer detectable as built-ins:

Builtin.typeOf(new Uint8Array(0));      // 'Uint8Array'

Uint8Array[Symbol.builtin] = undefined;

Builtin.typeOf(new Uint8Array(0));      // 'object'

The @@builtin property has the attributes:

  • [[Configurable]]: true
  • [[Enumerable]]: false
  • [[Writable]]: true

Abstract Operations


The abstract operation GetBuiltinValue with argument object performs the following steps:

  • Let fn be ? GetMethod(object, @@builtin).
  • If fn is undefined, return undefined.
  • Let value be ? Call(fn, object).
  • If value is undefined, return undefined.
  • Return ? ToString(value).


The abstract operation GetOwnBuiltinValue with argument object performs the following steps:

  • Let hasProperty be ? HasOwnProperty(object, @@builtin).
  • If hasProperty is false, return undefined.
  • Return ? GetBuiltinValue(object).


The Builtin object is the %Builtin% intrinsic object and the initial value of the Builtin property of the global object. The Builtin object is an ordinary object.

The value of the [[Prototype]] internal slot of the Builtin object is the intrinsic object %ObjectPrototype%.

The Builtin object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use the Builtin object as a constructor with the new operator. The Builtin object also does not have a [[Call]] internal method; it is not possible to invoke the Builtin object as a function., value2)

When called with arguments value1 and value2:

  • If Type(value1) is not Object return false.
  • Let V1 be ? GetOwnBuiltinValue(value1).
  • If V1 is undefined, return false.
  • If value2 is undefined, return false.
  • If Type(value2) is not Object, return false.
  • Let V2 be ? GetOwnBuiltinValue(value2).
  • Let same be the result of performing Strict Equality Comparison V1 === V2.
  • Return same

The function returns true if both of the given values have a @@builtin own property function that each returns values that, after coercion to a string, are strictly equal to one another. Otherwise, return false., vm.runInNewContext('Date'));     // true, vm.runInNewContext('Number'));   // false, vm.runInNewContext('{}'));       // false{}, vm.runInNewContext('{}'));         // false

Date = {};, vm.runInNewContext('Date'));     // false

Note that user code may modify the @@builtin own property on any object:

Date[Symbol.builtin] = undefined;, vm.runInNewContext('Date'));     // false

By default, the function will not throw an exception. It is possible for to throw if a user-provided @@builtin function throws or returns a value that cannot be coerced to a string (e.g. Symbol values).


When the typeOf() function is called with argument arg:

  • If Type(arg) is Object, then:
    • Let C be ? Get(arg, "constructor").
    • If C is not undefined, then:
      • Let V be ? GetBuiltinValue(C).
      • If V is not undefined, return V.
  • Return typeof arg.

For example:

Builtin.typeOf([]);                             // 'Array'
Builtin.typeOf(new ArrayBuffer());              // 'ArrayBuffer'
Builtin.typeOf(async function foo() {});        // 'AsyncFunction'
Builtin.typeOf(new Boolean());                  // 'Boolean'
Builtin.typeOf(new DataView(buffer));           // 'DataView'
Builtin.typeOf(new Date());                     // 'Date'
Builtin.typeOf(new Error());                    // 'Error'
Builtin.typeOf(new EvalError());                // 'EvalError'
Builtin.typeOf(new Float32Array());             // 'Float32Array'
Builtin.typeOf(new Float64Array());             // 'Float64Array'
Builtin.typeOf(function() {});                  // 'function'
Builtin.typeOf(function*() {});                 // 'GeneratorFunction'
Builtin.typeOf(new Int16Array());               // 'Int16Array'
Builtin.typeOf(new Int32Array());               // 'Int32Array'
Builtin.typeOf(new Int8Array());                // 'Int8Array'
Builtin.typeOf(new InternalError());            // 'InternalError'
Builtin.typeOf(new Intl.Collator());            // 'Collator'
Builtin.typeOf(new Intl.DateTimeFormat());      // 'DateTimeFormat'
Builtin.typeOf(new Intl.NumberFormat());        // 'NumberFormat'
Builtin.typeOf(new Map());                      // 'Map'
Builtin.typeOf(new Number());                   // 'Number'
Builtin.typeOf(new Object());                   // 'object'
Builtin.typeOf(new Promise(() => {}));          // 'Promise'
Builtin.typeOf(new RangeError());               // 'RangeError'
Builtin.typeOf(new ReferenceError());           // 'ReferenceError'
Builtin.typeOf(new RegExp(''));                 // 'RegExp'
Builtin.typeOf(new Set());                      // 'Set'
Builtin.typeOf(new SharedArrayBuffer());        // 'SharedArrayBuffer'
Builtin.typeOf(new String());                   // 'String'
Builtin.typeOf(new SyntaxError());              // 'SyntaxError'
Builtin.typeOf(new TypeError());                // 'TypeError'
Builtin.typeOf(new URIError());                 // 'URIError'
Builtin.typeOf(new Uint16Array());              // 'Uint16Array'
Builtin.typeOf(new Uint32Array());              // 'Uint32Array'
Builtin.typeOf(new Uint8Array());               // 'Uint8Array'
Builtin.typeOf(new Uint8ClampedArray());        // 'Uint8ClampedArray'
Builtin.typeOf(new WeakMap());                  // 'WeakMap'
Builtin.typeOf(new WeakSet());                  // 'WeatSet'
Builtin.typeOf(new WebAssembly.Module());       // 'Module'
Builtin.typeOf(new WebAssembly.Instance());     // 'Instance'
Builtin.typeOf(new WebAssembly.Memory());       // 'Memory'
Builtin.typeOf(new WebAssembly.Table());        // 'Table'
Builtin.typeOf(new WebAssembly.CompileError()); // 'CompileError'
Builtin.typeOf(new WebAssembly.LinkError());    // 'LinkError'
Builtin.typeOf(new WebAssembly.RuntimeError()); // 'RuntimeError'
Builtin.typeOf(null);                           // 'null'
Builtin.typeOf(undefined);                      // 'undefined'
Builtin.typeOf({});                             // 'object'
Builtin.typeOf(true);                           // 'boolean'
Builtin.typeOf(1);                              // 'number'
Builtin.typeOf('test');                         // 'string'
Builtin.typeOf(Symbol('foo'));                  // 'symbol'
Builtin.typeOf(function() {});                  // 'function'

class MyArray extends Uint8Array {}
const myArray = new MyArray();
Builtin.typeOf(myArray);                        // 'Uint8Array'

vm.runInNewContext('Builtin.typeOf(myArray)', { myArray }); // 'Uint8Array'

By default, the Builtin.typeOf() function will not throw an exception. It is possible for Builtin.typeOf() to throw if a user-provided @@builtin function throws or returns a value that cannot be coerced to a string (e.g. Symbol values).

Note: Because of the nature of Proxy instances, it is not possible for Builtin.typeOf(proxyObj) to ever return 'Proxy'.


Returns true if value is a Proxy exotic object, otherwise return false.

The Proxy.isProxy() function will not throw an exception.

Note: Due to the security issues around Proxy, host environments should be allowed to provide an option for forcing Proxy.isProxy(value) to always return false. For instance, Node.js could hypothetically provide a command-line argument like --disable-isproxy.


  • Adding a new %Builtin% intrinsic object can be avoided by adding functions to an existing intrinisic, for instance Object.isBuiltin() or Object.typeOf().

  • Using @@builtin means that any object can lie about being a built-in by setting the @@builtin own property to whatever value it wants. This is by design. Polyfills/shims and secure-realm code, for example, must be able to create builtins, remove them, or replace builtins that are noncompliant - as such, a shim (that runs before other code) must be able to create its own builtin replacement and truly masquerade as if it were the original builtin.

  • Why have a separate Proxy.isProxy() function? For the simple reason that Proxy objects do not act like anything else. The use case justifying Proxy.isProxy() is that, when debugging, it can often be necessary to know if the an object of interest is a Proxy or not.

  • The Builtin property on the global object is set initially to the Builtin object. This property has the attributes:

    • [[Configurable]]: true
    • [[Enumerable]]: true
    • [[Writable]]: true
  • The, Builtin.typeOf, and Proxy.isProxy properties have the attributes:

    • [[Configurable]]: true
    • [[Enumerable]]: true
    • [[Writable]]: true


function formatValue(value) {
  switch (Builtin.typeOf(value)) {
    case 'Date':
      return formatDate(value);
    case 'Array':
      return formatArray(value);
    case 'RegExp':
      return formatRegExp(value);
    /** ... **/
const val = vm.runInNewContext('Date');
if (, Date)) {
  /** ... **/
} else if (, Math)) {
  /** ... **/

Because the value of @@builtin is a function, the original implementation can be captured, cached, and restored later:

const origDateBuiltin = Date[Symbol.builtin];
Date[Symbol.builtin] = undefined;, vm.runInNewContext('Date'));  // false
Builtin.typeOf(Date);                          // 'object';                    // 'Date'

Date[Symbol.builtin] = origDateBuiltin;, vm.runInNewContext('Date'));  // true
Builtin.typeOf(Date);                          // 'Date'

Note: The behavior of the initial @@builtin function is to return the value of the this objects [[Builtin]] internal slot if one exists. Accordingly, it is possible to grab a reference to the function once and use it on multiple objects:

const origBuiltin = Date[Symbol.builtin];
Uint8Array[Symbol.builtin] = origBuiltin;

class Foo {}
Foo[Symbol.builtin] = origBuiltin;

Date[Symbol.builtin]();                                  // 'Date'
Uint8Array[Symbol.builtin]();                            // 'Uint8Array'
Foo[Symbol.builtin]();                                   // undefined, vm.runInNewContext('Date'));            // true, vm.runInNewContext('Uin8Array')); // true

Builtin.typeOf(new Date());                              // 'Date'
Builtin.typeOf(new Uint8Array());                        // 'Uint8Array'
Builtin.typeOf(new Foo());                               // 'object'