Authors:

• Robin Ricard (Bloomberg)
• Rick Button (Bloomberg)
• Nicolò Ribaudo (Babel)

Champions:

• Robin Ricard (Bloomberg)
• Rick Button (Bloomberg)

Advisors

• Philipp Dunkel (Bloomberg)
• Dan Ehrenberg (Igalia)
• Maxwell Heiber

Stage: 2

This proposal introduces two new deeply immutable data structures to JavaScript:

• Record, a deeply immutable Object-like structure #{ x: 1, y: 2 }
• Tuple, a deeply immutable Array-like structure #[1, 2, 3, 4]

Records and Tuples can only contain primitives and other Records and Tuples. You could think of Records and Tuples as "compound primitives". By being thoroughly based on primitives, not objects, Records and Tuples are deeply immutable.

Additionally, this proposal introduces a third new primitive type (Box) to ergonomically store object references inside Records and Tuples: #{ prop: Box(object) }.

Records and Tuples support comfortable idioms for construction, manipulation and use, similar to working with objects and Arrays. They are compared deeply by their contents, rather than by their identity.

JavaScript engines may perform certain optimizations on construction, manipulation and comparison of Records and Tuples, analogous to the way Strings are often implemented in JS engines. (It should be understood that these optimizations are not guaranteed.)

Records and Tuples aim to be usable and understood with external typesystem supersets such as TypeScript or Flow.

Today, userland libraries implement similar concepts, such as Immutable.js. Also a previous proposal has been attempted but abandoned because of the complexity of the proposal and lack of sufficient use cases.

This new proposal is still inspired by this previous proposal but introduces some significant changes: Record and Tuples are now deeply immutable. This property is fundamentally based on the observation that, in large projects, the risk of mixing immutable and mutable data structures grows as the amount of data being stored and passed around grows as well so you'll be more likely handling large record & tuple structures. This can introduce hard-to-find bugs.

As a built-in, deeply immutable data structure, this proposal also offers a few usability advantages compared to userland libraries:

• Records and Tuples are easily introspectable in a debugger, while library provided immutable types are often hard to inspect as you have to inspect through data structure details.
• Because they're accessed through typical object and array idioms, no additional branching is needed in order to write a generic library that consumes both immutable and JS objects; with user libraries, method calls may be needed just in the immutable case.
• We avoid cases where developers may expensively convert between regular JS objects and immutable structures, by making it easier to just always use the immutable ones.

Immer is a notable approach to immutable data structures, and prescribes a pattern for manipulation through producers and reducers. It is not providing immutable data types however, as it generates frozen objects. This same pattern can be adapted to the structures defined in this proposal in addition to frozen objects.

Deep equality as defined in user libraries can vary significantly, in part due to possible references to mutable objects. By drawing a hard line about only deeply containing primitives, Records and Tuples, and recursing through the entire structure, this proposal defines simple, unified semantics for comparisons.

const proposal = #{
  id: 1234,
  title: "Record & Tuple proposal",
  contents: `...`,
  // tuples are primitive types so you can put them in records:
  keywords: #["ecma", "tc39", "proposal", "record", "tuple"],
};

// Accessing keys like you would with objects!
console.log(proposal.title); // Record & Tuple proposal
console.log(proposal.keywords[1]); // tc39

// Spread like objects!
const proposal2 = #{
  ...proposal,
  title: "Stage 2: Record & Tuple",
};
console.log(proposal2.title); // Stage 2: Record & Tuple
console.log(proposal2.keywords[1]); // tc39

// Object functions work on Records:
console.log(Object.keys(proposal)); // ["contents", "id", "keywords", "title"]

Open in playground

Functions can handle Records and Objects in generally the same way:

const ship1 = #{ x: 1, y: 2 };
// ship2 is an ordinary object:
const ship2 = { x: -1, y: 3 };

function move(start, deltaX, deltaY) {
  // we always return a record after moving
  return #{
    x: start.x + deltaX,
    y: start.y + deltaY,
  };
}

const ship1Moved = move(ship1, 1, 0);
// passing an ordinary object to move() still works:
const ship2Moved = move(ship2, 3, -1);

console.log(ship1Moved === ship2Moved); // true
// ship1 and ship2 have the same coordinates after moving

Open in playground

See more examples here.

const measures = #[42, 12, 67, "measure error: foo happened"];

// Accessing indices like you would with arrays!
console.log(measures[0]); // 42
console.log(measures[3]); // measure error: foo happened

// Slice and spread like arrays!
const correctedMeasures = #[
  ...measures.slice(0, measures.length - 1),
  -1
];
console.log(correctedMeasures[0]); // 42
console.log(correctedMeasures[3]); // -1

// or use the .with() shorthand for the same result:
const correctedMeasures2 = measures.with(3, -1);
console.log(correctedMeasures2[0]); // 42
console.log(correctedMeasures2[3]); // -1

// Tuples support methods similar to Arrays
console.log(correctedMeasures2.map(x => x + 1)); // #[43, 13, 68, 0]

Open in playground

Similarly than with records, we can treat tuples as array-like:

const ship1 = #[1, 2];
// ship2 is an array:
const ship2 = [-1, 3];

function move(start, deltaX, deltaY) {
  // we always return a tuple after moving
  return #[
    start[0] + deltaX,
    start[1] + deltaY,
  ];
}

const ship1Moved = move(ship1, 1, 0);
// passing an array to move() still works:
const ship2Moved = move(ship2, 3, -1);

console.log(ship1Moved === ship2Moved); // true
// ship1 and ship2 have the same coordinates after moving

Open in playground

See more examples here.

const myObject = { x: 2 };

const record = #{
  name: "rec",
  data: Box(myObject)
};

console.log(record.data.unbox().x); // 2

// The box contents are classic mutable objects:
record.data.unbox().x = 3;
console.log(myObject.x); // 3

console.log(record === #{ name: "rec", data: Box(myObject) }); // true

Open in playground

As stated before Record & Tuple are deeply immutable: attempting to insert an object in them without first putting it in a Box will result in a TypeError:

const instance = new MyClass();
const constContainer = #{
    instance: instance
};
// TypeError: Record literals may only contain primitives, Records and Tuples

const tuple = #[1, 2, 3];

tuple.map(x => new MyClass(x));
// TypeError: Callback to Tuple.prototype.map may only return primitives, Records or Tuples

// The following should work:
Array.from(tuple).map(x => new MyClass(x))

This defines the new pieces of syntax being added to the language with this proposal.

We define a record or tuple expression by using the # modifier in front of otherwise normal object or array expressions. There is no syntax to create box values.

#{}
#{ a: 1, b: 2 }
#{ a: 1, b: #[2, 3, #{ c: 4 }] }
#[]
#[1, 2]
#[1, 2, #{ a: 3 }]

Holes are prevented in syntax, unlike Arrays, which allow holes. See issue #84 for more discussion.

const x = #[,]; // SyntaxError, holes are disallowed by syntax

Using the __proto__ identifier as a property is prevented in syntax. See issue #46 for more discussion.

const x = #{ __proto__: foo }; // SyntaxError, __proto__ identifier prevented by syntax

const y = #{ ["__proto__"]: foo }; // valid, creates a record with a "__proto__" property.

Concise methods are disallowed in Record syntax.

#{ method() { } }  // SyntaxError

Records may only have String keys, not Symbol keys, due to the issues described in #15. Creating a Record with a Symbol key is a TypeError.

const record = #{ [Symbol()]: #{} };
// TypeError: Record may only have string as keys

Records and Tuples may only contain primitives and other Records and Tuples. Attempting to create a Record or Tuple that contains an Object (null is not an object) or a Function throws a TypeError.

const obj = {};
const record = #{ prop: obj }; // TypeError: Record may only contain primitive values
const record2 = #{ prop: Box(obj) }; // ok

Equality of Records and Tuples works like that of other JS primitive types like Boolean and String values, comparing by contents, not identity:

assert(#{ a: 1 } === #{ a: 1 });
assert(#[1, 2] === #[1, 2]);

This is distinct from how equality works for JS objects: comparison of objects will observe that each object is distinct:

assert({ a: 1 } !== { a: 1 });
assert(Object(#{ a: 1 }) !== Object(#{ a: 1 }));
assert(Object(#[1, 2]) !== Object(#[1, 2]));

Insertion order of record keys does not affect equality of records, because there's no way to observe the original ordering of the keys, as they're implicitly sorted:

assert(#{ a: 1, b: 2 } === #{ b: 2, a: 1 });

Object.keys(#{ a: 1, b: 2 })  // ["a", "b"]
Object.keys(#{ b: 2, a: 1 })  // ["a", "b"]

If their structure and contents are deeply identical, then Record and Tuple values considered equal according to all of the equality operations: Object.is, ==, ===, and the internal SameValueZero algorithm (used for comparing keys of Maps and Sets). They differ in terms of how -0 is treated:

• Object.is treats -0 and 0 as unequal
• ==, === and SameValueZero treat -0 with 0 as equal

Note that == and === are more direct about other kinds of values nested in Records and Tuples--returning true if and only if the contents are identical (with the exception of 0/-0). This directness has implications for NaN as well as comparisons across types. See examples below.

Two boxes are considered equal if they contain the same object:

const obj = {};
assert(Box(obj) === Box(obj));
assert(Box({}) !== Box({}));

See further discussion in #65.

assert(#{ a:  1 } === #{ a: 1 });
assert(#[1] === #[1]);

assert(#{ a: -0 } === #{ a: +0 });
assert(#[-0] === #[+0]);
assert(#{ a: NaN } === #{ a: NaN });
assert(#[NaN] === #[NaN]);

assert(#{ a: -0 } == #{ a: +0 });
assert(#[-0] == #[+0]);
assert(#{ a: NaN } == #{ a: NaN });
assert(#[NaN] == #[NaN]);
assert(#[1] != #["1"]);

assert(!Object.is(#{ a: -0 }, #{ a: +0 }));
assert(!Object.is(#[-0], #[+0]));
assert(Object.is(#{ a: NaN }, #{ a: NaN }));
assert(Object.is(#[NaN], #[NaN]));

// Map keys are compared with the SameValueZero algorithm
assert(new Map().set(#{ a: 1 }, true).get(#{ a: 1 }));
assert(new Map().set(#[1], true).get(#[1]));
assert(new Map().set(#[-0], true).get(#[0]));

In general, you can treat Records like objects. For example, the Object namespace and the in operator work with Records.

const keysArr = Object.keys(#{ a: 1, b: 2 }); // returns the array ["a", "b"]
assert(keysArr[0] === "a");
assert(keysArr[1] === "b");
assert(keysArr !== #["a", "b"]);
assert("a" in #{ a: 1, b: 2 });

JS developers will typically not have to think about Record and Tuple wrapper objects, but they're a key part of how Records and Tuples work "under the hood" in the JavaScript specification.

Accessing of a Record or Tuple via . or [] follows the typical GetValue semantics, which implicitly converts to an instance of the corresponding wrapper type. You can also do the conversion explicitly through Object():

• Object(record) creates a Record wrapper object
• Object(tuple) creates a Tuple wrapper object

(One could imagine that new Record or new Tuple could create these wrappers, like new Number and new String do, but Records and Tuples follow the newer convention set by Symbol and BigInt, making these cases throw, as it's not the path we want to encourage programmers to take.)

Record and Tuple wrapper objects have all of their own properties with the attributes writable: false, enumerable: true, configurable: false. The wrapper object is not extensible. All put together, they behave as frozen objects. This is different from existing wrapper objects in JavaScript, but is necessary to give the kinds of errors you'd expect from ordinary manipulations on Records and Tuples.

An instance of Record has the same keys and values as the underlying record value. The __proto__ of each of these Record wrapper objects is null (discussion: #71).

An instance of Tuple has keys that are integers corresponding to each index in the underlying tuple value. The value for each of these keys is the corresponding value in the original tuple. In addition, there is a non-enumerable length key. Overall, these properties match those of the String wrapper object. That is, Object.getOwnPropertyDescriptors(Object(#["a", "b"])) and Object.getOwnPropertyDescriptors(Object("ab")) each return an object that looks like this:

{
  "0": {
    "value": "a",
    "writable": false,
    "enumerable": true,
    "configurable": false
  },
  "1": {
    "value": "b",
    "writable": false,
    "enumerable": true,
    "configurable": false
  },
  "length": {
    "value": 2,
    "writable": false,
    "enumerable": false,
    "configurable": false
  }
}

The __proto__ of Tuple wrapper objects is Tuple.prototype. Note that, if you're working across different JavaScript global objects ("Realms"), the Tuple.prototype is selected based on the current Realm when the Object conversion is performed, similarly to how the .prototype of other primitives behaves -- it's not attached to the Tuple value itself. Tuple.prototype has various methods on it, analogous to Arrays.

For integrity, out-of-bounds numerical indexing on Tuples returns undefined, rather than forwarding up through the prototype chain, as with TypedArrays. Lookup of non-numerical property keys forwards up to Tuple.prototype, which is important to find their Array-like methods.

Tuple values have functionality broadly analogous to Array. Similarly, Record values are supported by different Object static methods.

assert(Object.keys(#{ a: 1, b: 2 }) === #["a", "b"]);
assert(#[1, 2, 3].map(x => x * 2), #[2, 4, 6]);

See the appendix to learn more about the Record & Tuple namespaces.

You can convert structures using Record(), Tuple() (with the spread operator), Record.fromEntries() or Tuple.from():

const record = Record({ a: 1, b: 2, c: 3 });
const record2 = Record.fromEntries([#["a", 1], #["b", 2], #["c", 3]]); // note that an iterable will also work
const tuple = Tuple(...[1, 2, 3]);
const tuple2 = Tuple.from([1, 2, 3]); // note that an iterable will also work

assert(record === #{ a: 1, b: 2, c: 3 });
assert(tuple === #[1, 2, 3]);
Record.from({ a: {} }); // TypeError: Can't convert Object with a non-const value to Record
Tuple.from([{}, {} , {}]); // TypeError: Can't convert Iterable with a non-const value to Tuple

Note that Record(), Tuple(), Record.fromEntries() and Tuple.from() expect collections consisting of Records, Tuples or other primitives (such as Numbers, Strings, etc). Nested object references would cause a TypeError. It's up to the caller to convert inner structures in whatever way is appropriate for the application.

Note: The current draft proposal does not contain recursive conversion routines, only shallow ones. See discussion in #122

Like Arrays, Tuples are iterable.

const tuple = #[1, 2];

// output is:
// 1
// 2
for (const o of tuple) { console.log(o); }

Similarly to Objects, Records are only iterable in conjunction with APIs like Object.entries.

const record = #{ a: 1, b: 2 };

// TypeError: record is not iterable
for (const o of record) { console.log(o); }

// Object.entries can be used to iterate over Records, just like for Objects
// output is:
// a
// b
for (const [key, value] of Object.entries(record)) { console.log(key) }

• The behavior of JSON.stringify(record) is equivalent to calling JSON.stringify on the object resulting from recursively converting the record to an object that contains no records or tuples.
• The behavior of JSON.stringify(tuple) is equivalent to calling JSON.stringify on the array resulting from recursively converting the tuple to an array that contains no records or tuples.

JSON.stringify(#{ a: #[1, 2, 3] }); // '{"a":[1,2,3]}'
JSON.stringify(#[true, #{ a: #[1, 2, 3] }]); // '[true,{"a":[1,2,3]}]'

We propose to add JSON.parseImmutable so we can extract a Record/Tuple type out of a JSON string instead of an Object/Array.

The signature of JSON.parseImmutable is identical to JSON.parse with the only change being in the return type that is now a Record or a Tuple.

Tuple supports instance methods similar to Array with a few changes:

• The mechanics of Tuple and Array methods are a bit different; Array methods generally depend on being able to incrementally modify the Array, and are built for subclassing, neither of which would apply for Tuples.
• Operations which mutate the Array are not supported. For example, there is no Tuple.prototype.push method.
• Tuples include the methods introduced by the Change Array by copy proposal, such as Tuple.prototype.withAt.

The appendix contains a full description of Tuple's prototype.

typeof identifies Records, Tuples and Boxes as distinct types:

assert(typeof #{ a: 1 } === "record");
assert(typeof #[1, 2]   === "tuple");
assert(typeof Box({}) === "box");

It is possible to use a Record, Tuple or Box as a key in a Map, and as a value in a Set. When using a Record or Tuple here, they are compared by value.

It is not possible to use a Record or Tuple as a key in a WeakMap or as a value in a WeakSet, because Records and Tuples are not Objects, and their lifetime is not observable.

const record1 = #{ a: 1, b: 2 };
const record2 = #{ a: 1, b: 2 };

const map = new Map();
map.set(record1, true);
assert(map.get(record2));

const record1 = #{ a: 1, b: 2 };
const record2 = #{ a: 1, b: 2 };

const set = new Set();
set.add(record1);
set.add(record2);
assert(set.size === 1);

const record = #{ a: 1, b: 2 };
const weakMap = new WeakMap();

// TypeError: Can't use a Record as the key in a WeakMap
weakMap.set(record, true);

const record = #{ a: 1, b: 2 };
const weakSet = new WeakSet();

// TypeError: Can't add a Record to a WeakSet
weakSet.add(record);

One core benefit of the Records and Tuples proposal is that they are compared by their contents, not their identity. At the same time, === in JavaScript on objects has very clear, consistent semantics: to compare the objects by identity. Making Records and Tuples primitives enables comparison based on their values.

At a high level, the object/primitive distinction helps form a hard line between the deeply immutable, context-free, identity-free world and the world of mutable objects above it. This category split makes the design and mental model clearer.

An alternative to implementing Record and Tuple as primitives would be to use operator overloading to achieve a similar result, by implementing an overloaded abstract equality (==) operator that deeply compares objects. While this is possible, it doesn't satisfy the full use case, because operator overloading doesn't provide an override for the === operator. We want the strict equality (===) operator to be a reliable check of "identity" for objects and "observable value" (modulo -0/+0/NaN) for primitive types.

Another option is to perform what is called interning: we track globally Record or Tuple objects and if we attempt to create a new one that happens to be identical to an existing Record object, we now reference this existing Record instead of creating a new one. This is essentially what the polyfill does. We're now equating value and identity. This approach creates problems once we extend that behavior across multiple JavaScript contexts and wouldn't give deep immutability by nature and it is particularly slow which would make using Record & Tuple a performance-negative choice.

Record & Tuple is built to interoperate with objects and arrays well: you can read them exactly the same way as you would do with objects and arrays. The main change lies in the deep immutability and the comparison by value instead of identity.

Developers used to manipulating objects in an immutable manner (such as transforming pieces of Redux state) will be able to continue to do the same manipulations they used to do on objects and arrays, this time, with more guarantees.

We are going to do empirical research through interviews and surveys to figure out if this is working out as we think it does.

If we want to keep access to Record & Tuple similar to Objects and Arrays as described in the previous section, we can't rely on methods to perform that access. Doing so would require us to branch code when trying to create a "generic" function able to take Objects/Arrays/Records/Tuples.

Here is an example function that has support for Immutable.js Records and ordinary objects:

const ProfileRecord = Immutable.Record({
    name: "Anonymous",
    githubHandle: null,
});

const profileObject = {
    name: "Rick Button",
    githubHandle: "rickbutton",
};
const profileRecord = ProfileRecord({
    name: "Robin Ricard",
    githubHandle: "rricard",
});

function getGithubUrl(profile) {
    if (Immutable.Record.isRecord(profile)) {
        return `https://github.com/${
            profile.get("githubHandle")
        }`;
    }
    return `https://github.com/${
        profile.githubHandle
    }`;
}

console.log(getGithubUrl(profileObject)) // https://github.com/rickbutton
console.log(getGithubUrl(profileRecord)) // https://github.com/rricard

This is error-prone as both branches could easily get out of sync over time...

Here is how we would write that function taking Records from this proposal and ordinary objects:

const profileObject = {
  name: "Rick Button",
  githubHandle: "rickbutton",
};
const profileRecord = #{
  name: "Robin Ricard",
  githubHandle: "rricard",
};

function getGithubUrl(profile) {
  return `https://github.com/${
    profile.githubHandle
  }`;
}

console.log(getGithubUrl(profileObject)) // https://github.com/rickbutton
console.log(getGithubUrl(profileRecord)) // https://github.com/rricard

This function support both Objects and Records in a single code-path as well as not forcing the consumer to choose which data structures to use.

Why do we need to support both at the same time anyway? This is primarily to avoid an ecosystem split. Let's say we're using Immutable.js to do our state management but we need to feed our state to a few external libraries that don't support it:

state.jobResult = Immutable.fromJS(
    ExternalLib.processJob(
        state.jobDescription.toJS()
    )
);

Both toJS() and fromJS() can end up being very expensive operations depending on the size of the substructures. An ecosystem split means conversions that, in turn, means possible performance issues.

The proposed syntax significantly improves the ergonomics of using Record and Tuple in code. For example:

// with the proposed syntax
const record = #{
  a: #{
    foo: "string",
  },
  b: #{
    bar: 123,
  },
  c: #{
    baz: #{
      hello: #[
        1,
        2,
        3,
      ],
    },
  },
};

// with only the Record/Tuple globals
const record = Record({
  a: Record({
    foo: "string",
  }),
  b: Record({
    bar: 123,
  }),
  c: Record({
    baz: Record({
      hello: Tuple(
        1,
        2,
        3,
      ),
    }),
  }),
});

The proposed syntax is intended to be simpler and easier to understand, because it is intentionally similar to syntax for object and array literals. This takes advantage of the user's existing familiarity with objects and arrays. Additionally, the second example introduces additional temporary object literals, which adds to complexity of the expression.

Using a keyword as a prefix to the standard object/array literal syntax presents issues around backwards compatibility. Additionally, re-using existing keywords can introduce ambiguity.

ECMAScript defines a set of reserved keywords that can be used for future extensions to the language. Defining a new keyword that is not already reserved is theoretically possible, but requires significant effort to validate that the new keyword will not likely break backwards compatibility.

Using a reserved keyword makes this process easier, but it is not a perfect solution because there are no reserved keywords that match the "intent" of the feature, other than const. The const keyword is also tricky, because it describes a similar concept (variable reference immutability) while this proposal intends to add new immutable data structures. While immutability is the common thread between these two features, there has been significant community feedback that indicates that using const in both contexts is undesirable.

Instead of using a keyword, {| |} and [||] have been suggested as possible alternatives. Currently, the champion group is leaning towards #[]/#{}, but discussion is ongoing in #10.

The definition of Record & Tuple as compound primitives forces everything in Record & Tuple to not be objects. This comes with some drawbacks (referencing objects becomes harder but is still possible) but also more guarantees to avoid common programming mistakes.

const object = {
   a: {
       foo: "bar",
   },
};
Object.freeze(object);
func(object);

// func is able to mutate object’s keys even if object is frozen

In the above example, we try to create a guarantee of immutability with Object.freeze. Unfortunately, since we did not freeze the object deeply, nothing tells us that object.a has been untouched. With Record & Tuple that constraint is by nature and there is no doubt that the structure is untouched:

const record = #{
   a: #{
       foo: "bar",
   },
};
func(record);
// runtime guarantees that record is entirely unchanged
assert(record.a.foo === "bar");

Finally, deep immutability suppresses the need for a common pattern which consists of deep-cloning objects to keep guarantees:

const clonedObject = JSON.parse(JSON.stringify(object));
func(clonedObject);
// now func can have side effects on clonedObject, object is untouched
// but at what cost?
assert(object.a.foo === "bar");

In general, the spread operator works well for this:

// Add a Record field
let rec = #{ a: 1, x: 5 }
#{ ...rec, b: 2 }  // #{ a: 1, b: 2, x: 5 }

// Change a Record field
#{ ...rec, x: 6 }  // #{ a: 1, x: 6 }

// Append to a Tuple
let tup = #[1, 2, 3];
#[...tup, 4]  // #[1, 2, 3, 4]

// Prepend to a Tuple
#[0, ...tup]  // #[0, 1, 2, 3]

// Prepend and append to a Tuple
#[0, ...tup, 4]  // #[0, 1, 2, 3, 4]

And if you're changing something in a Tuple, the Tuple.prototype.with method works:

// Change a Tuple index
let tup = #[1, 2, 3];
tup.with(1, 500)  // #[1, 500, 3]

Some manipulations of "deep paths" can be a bit awkward. For that, the Deep Path Properties for Records proposal adds additional shorthand syntax to Record literals.

We are developing the deep path properties proposal as a separate follow-on proposal because we don't see it as core to using Records, which work well independently. It's the kind of syntactic addition which would work well to prototype over time in transpilers, and where we have many decision points which don't have to do with Records and Tuples (e.g., how it works with objects).

We've talked with the Readonly Collections champions, and both groups agree that these are complements:

• Readonly collections are shallowly immutable and may point to objects; they may be mutated during construction, and read-only views of mutating objects are supported.
• Records and Tuples are deeply immutable and consist only of primitives.

Neither one is a subset of the other in terms of functionality. At best, they are parallel, just like each proposal is parallel to other collection types in the language.

So, the two champion groups have resolved to ensure that the proposals are in parallel with respect to each other. For example, this proposal adds a new Tuple.prototype.withReversed method. The idea would be to check, during the design process, if this signature would also make sense for read-only Arrays (if those exist): we extracted these new methods to the Change Array by copy proposal, so that we can discuss an API which builds a consistent, shared mental model.

In the current proposal drafts, there aren't any overlapping types for the same kind of data, but both proposals could grow in these directions in the future, and we're trying to think these things through ahead of time. Who knows, some day TC39 could decide to add primitive RecordMap and RecordSet types, as the deeply immutable versions of Set and Map! And these would be in parallel with Readonly Collections types.

TC39 has been long discussing "value types", which would be some kind of class declaration for a primitive type, for several years, on and off. An earlier version of this proposal even made an attempt. This proposal tries to start off simple and minimal, providing just the core structures. The hope is that it could provide the data model for a future proposal for classes.

This proposal is loosely related to a broader set of proposals, including operator overloading and extended numeric literals: These all conspire to provide a way for user-defined types to do the same as BigInt. However, the idea is to add these features if we determine they're independently motivated.

If we had user-defined primitive/value types, then it could make sense to use them in built-in features, such as CSS Typed OM or the Temporal Proposal. However, this is far in the future, if it ever happens; for now, it works well to use objects for these sorts of features.

Although both kinds of Records relate to Objects, and both kinds of Tuples relate to Arrays, that's about where the similarity ends.

Records in TypeScript are a generic utility type to represent an object taking a key type matching with a value type. They still represent objects.

Likewise, Tuples in TypeScript are a notation to express types in an array of a limited size (starting with TypeScript 4.0 they have a variadic form). Tuples in TypeScript are a way to express arrays with heterogeneous types. ECMAScript tuples can correspond to TS arrays or TS tuples easily as they can either contain an indefinite number of values of the same type or contain a limited number of values with different types.

TS Records or Tuples are orthogonal features to ECMAScript Records and Tuples and both could be expressed at the same time:

const record: readonly Record<string, number> = #{
  foo: 1,
  bar: 2,
};
const tuple: readonly [number, string] = #[1, "foo"];

This proposal does not make any performance guarantees and does not require specific optimizations in implementations. Based on feedback from implementers, it is expected that they will implement common operations via "linear time" algorithms. However, this proposal does not prevent some classical optimizations for purely functional data structures, including but not limited to:

• Optimizations for making deep equality checks fast:
  • For returning true quickly, intern ("hash-cons") some data structures
  • For returning false quickly, maintain a hash of the tree of the contents of some structures
• Optimizations for manipulating data structures
  • In some cases, reuse existing data structures (e.g., when manipulated with object spread), similar to ropes or typical implementations of functional data structures
  • In other cases, as determined by the engine, use a flat representation like existing JavaScript object implementations

These optimizations are analogous to the way that modern JavaScript engines handle string concatenation, with various different internal types of strings. The validity of these optimizations rests on the unobservability of the identity of records and tuples. It's not expected that all engines will act identically with respect to these optimizations, but rather, they will each make decisions about which particular heuristics to use. Before Stage 4 of this proposal, we plan to publish a guide for best practices for cross-engine optimizable use of Records and Tuples, based on the implementation experience that we will have at that point.

A new, deeply immutable, compound primitive type data structure, proposed in this document, that is analogous to Object. #{ a: 1, b: 2 }

A new, deeply immutable, compound primitive type data structure, proposed in this document, that is analogous to Array. #[1, 2, 3, 4]

A new primitive type that can store a reference to an object, proposed in this document. Box({}).

Values which act like other JavaScript primitives, but are composed of other constituent values. This document proposes the first three compound primitive types: Record, Tuple and Box.

String, Number, Boolean, undefined, null, Symbol and BigInt

Things which are either compound or simple primitive types. All primitives in JavaScript share certain properties:

• They are deeply immutable
• Comparison is by value, not by identity
• They are not objects in the object model--object operations lead to exceptions or implicit wrapper creation.

A data structure that doesn't accept operations that change it internally, but instead has operations that return a new value that is the result of applying that operation to it.

In this proposal Record and Tuple are deeply immutable data structures.

The operator === is defined with the Strict Equality Comparison algorithm. Strict Equality refers to this particular notion of equality.

Structural sharing is a technique used to limit the memory footprint of immutable data structures. In a nutshell, when applying an operation to derive a new version of an immutable structure, structural sharing will attempt to keep most of the internal structure intact and used by both the old and derived versions of that structure. This greatly limits the amount to copy to derive the new structure.