How to use promise in jerryscript?

[edword01]

when create promise in C file, like this: E.g.:

jerry_value_t test_promise = jerry_create_promise();
jerry_value_t argument = jerry_create_string((jerry_char_t*)"promise");
 jerry_value_t is_ok = jerry_resolve_or_reject_promise (test_promise,
                                                                    argument,
                                                                    true);

and then, how to get this promise in js?? where to find an example?

[zherczeg]

There are many ways to pass any value to ES world. You can use it for any value, not just promises. Some examples:

• Return the value from an external function: https://github.com/jerryscript-project/jerryscript/blob/master/docs/02.API-REFERENCE.md#jerry_create_external_function
• Get a callback, and call it with https://github.com/jerryscript-project/jerryscript/blob/master/docs/02.API-REFERENCE.md#jerry_call_function
• Add it as a property for an existing object (global object, an object passed to an external callback, etc.) https://github.com/jerryscript-project/jerryscript/blob/master/docs/02.API-REFERENCE.md#jerry_set_property

Don't forget about reference counting.

[edword01]

Not only pass value to ES world, I want to get the sensor value from C , and do this job in JS like event listener. so Promise object can use Promise.then to handle this . now i set several task in RTOS,

TaskA: call jerry_init, mount_spifff, .... ....... ,, but doesn't create promise object.

TaskB: call func jerry_create_promise , and jerry_resolve_or_reject_promise, like above, in every 3seconds.

now in JS file , how to quote this promise obj create in C ? Don't know the method or gramma for jerryscript

[zherczeg]

Don't forget JerryScript is not thread safe, so accessing the API from multiple threads at the same time will likely lead to crash.

Your workflow should look like this:

• Have a JerryScript thread, which runs your ES code
• Create a "current promise" which will be resolved when the event occures
• Create an external function, which returns with this "current promise" (don't forget to increase its refcount). The ES code can call this external function to get the "current promise"
• When the event occures, notify the JerryScript thread
• The JerryScript thread resolves the "current promise", release it and creates a new current one (this step can be delayed until the external function is actually called)
• Don't forget to run the jobqueue, otherwise promises don't work

[edword01]

what's the problem of this workflow: in rtos, have a jerryscript task, create external func "get_promise", create promise, load js file, then come into a while loop to wait a Semaphore , Semaphore is gived by another task, if it is gived, do the job of "jerry_resolve_or_reject_promise (promise_obj, argument, true); jerry_release_value(promise_obj); promise_obj = jerry_create_promise();"

loop like:

while(1) { ....... if(waitting the semaphore { //resolve promise, and create new one ; }

job_value = jerry_run_all_enqueued_jobs ();

//check job_value
..... }

and in the external functio "get_promise", just "return promise_obj",

In js file, var p = get_promise(); try { p.then(function(x){ console.log("promise"); }); }

All this crash now.

Crash info

0x3ffd87a0 0x400e410c: jmem_pools_alloc at ??:? 0x400de197: ecma_create_error_reference_from_context at ??:? 0x400daafd: jerry_resolve_or_reject_promise at ??:?

[zherczeg]

Your approach is correct. The crash might be caused by reference counting (e.g. not calling jerry_acquire_value in your get_promise external function). The jerry_resolve_or_reject_promise rarely throws an error, so the value passed to it might be invalid as well (e.g. a value which was freed)

[edword01]

So in get_promise func, use this?

jerry_value_t value = jerry_acquire_value(test_promise); return value;

in js file, promise still doesn't work.

[zherczeg]

yes, return jerry_acquire_value(test_promise)returns with another reference to the promise. There might be other errors in the code though.

[Sherry0901]

Have you solved the problem？ @edword01