[jira] [Created] (THRIFT-4406) JavaScript: Use modern Promise implementations

["Philip Frank (JIRA)" <ji...@apache.org>]

Philip Frank created THRIFT-4406:
------------------------------------

             Summary: JavaScript: Use modern Promise implementations
                 Key: THRIFT-4406
                 URL: https://issues.apache.org/jira/browse/THRIFT-4406
             Project: Thrift
          Issue Type: Improvement
          Components: JavaScript - Compiler
            Reporter: Philip Frank
            Priority: Minor


I would like to acces the results of service method invocations in a web browser environment using [Promises|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this] (or a compatible API). I'm willing to work on the necessary changes in the compiler and am seeking guidance regarding the best way to make these changes.

Current state:

In browser JS, i can pass a callback function as last argument when calling a method:

{code:javascript}
    service.getFoo('a1', 'a2', function(foo) {
        alert(foo);
    });
{code}


There is also a synchronous API, however this blocking way is discouraged by browser vendors. It also fails with an error for me when using WebSocketTransport instead of XHRTransport.

{code:javascript}
    alert(service.getFoo('a1', 'a2'));
{code}

In JS code generated for NodeJS, the 'q' library is included, which provides an API mostly compatible with JavaScript's native Promises.

{code:javascript}
    service.getFoo('a1', 'a2').then(function(foo) {
        alert(foo);
    });
{code}

From what I can tell 'q' is mostly irrelevant today since native Promises are supported in NodeJs [for many versions|http://node.green/#ES2015-built-ins-Promise].

Regarding support in browsers, native Promises work in [recent versions of most browsers|https://caniuse.com/#feat=promises] and can be enabled in other cases by including one of the many polyfill libraries, like the popular [core-js|https://github.com/zloirock/core-js#ecmascript-6-promise].


Proposed solution:

In both browser and nodeJS, service methods return a Promise when invoked without a callback. This has many advantages for developers:
* Promise API is an established standard in modern JavaScript (ECMAScript), it is easy for developers to consume and to use with other libraries
* Error handling is possible through {{catch()}}
* Result handling is very flexible through chaining
* Promises can be combined through {{Promise.all()}} and {{Promise.race()}}
* The [{{await}}|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await] keyword makes it even easier to write async code. (Support limited to some browsers or with cross-compilers like [babel|https://babeljs.io/])


However, this would break compatibility:
* Synchronous (blocking) invocations would no longer be supported, instead a Promise instead of the value is returned
* with NodeJS code relying on some features specific to 'q' (e.g. {{finally()}} and {{done()`)
* Internet Explorer needs a polyfill for Promises


I'm assuming the code generated by Thrift should stay backwards compatible in the foreseeable future, so I suggest making these changes behind a new compiler flag, {{-gen js:es6}} (for ECMAScript 6). This would also open up the ability to leverage more ES6 features in the future: For example, using {{const}} instead of {{var}} might give a slight performance improvement and using the {{class}} syntax could improve readability of the generated code.

Please share any thoughts regarding this topic or hints towards a good implementation. If there are no objections, I'm willing to start working on this soon, with the hopes of getting my changes accepted into the project codebase.



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