Proposal for the `composes` keyword

This is a proposal for the addition of the composes keyword into the ECMAScript programming language spec.


The composes keyword adds support for composition (as in composition over inheritance) in ECMAScript classes.

class LoginForm composes Validation, AsyncIO { ... }

The proposed syntax is compatible with existing keywords as well as concepts in superset languages such as Typescript.

class LoginForm extends Layout composes Validation, AsyncIO implements ContractInterface { ... }

Prior work

Modern web frameworks often use similar concepts in one form or another:

Composition is often championed by miscellaneous authors: 1, 2, 3


As shown above, many libraries and authors are actively working to support composition in a number of alternative ways.

Currently, composition is often achieved via nested higher order components:

withAsyncIO(withValidation(class LoginForm { ... }))

The proposed syntax improves clarity and readability considerably:

class LoginForm composes Validation, AsyncIO { ... }

Adding composition into the language spec as a first class citizen would offer clear benefits in maintainability and readability, as well as standardizing a currently fragmented problem space.

Limitations and issues

Composition brings numerous technical and design challenges, pertaining but not limited to scoping, this, etc.
At the current proposal stage, the author does not try to address them. In the future, already existing implementations can offer a good base for discussion.


Composition as a concept is already actively used by many libraries and authors, and championed at a considerable level.
Adding first class support for composition into the language itself would offer numerous clear wins for readability, maintainability, and more.

Could you please explain what your proposed composes keyword actually does (in terms of language semantics)?

The mixin use case for classes (like in that withValidation example) is probably solved by decorators already, at least syntactically.

And I'm not entirely sure after reading your post whether you are talking about object composition, function composition or something else?

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