[REF] doc: add async javascript code section

in javascript Cheatsheet, add a bunch of example about asynchronous code

closes #31716

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>

doc/reference/javascript_cheatsheet.rst

@@ -276,4 +276,268 @@ using the view in the kanban arch (a specific example is the helpdesk dashboard)
     Note: you can change the way the view interprets the arch structure.  However,
     from the server point of view, this is still a view of the same base type,
     subjected to the same rules (rng validation, for example).  So, your views still
-    need to have a valid arch field.
+    need to have a valid arch field.
+
+Promises and asynchronous code
+===============================
+For a very good and complete introduction to promises, please read this excellent article https://github.com/getify/You-Dont-Know-JS/blob/master/async%20%26%20performance/ch3.md
+
+Creating new Promises
+-----------------------
+- turn a constant into a promise
+    There are 2 static functions on Promise that create a resolved or rejected promise based on a constant:
+
+    .. code-block:: javascript
+
+        var p = Promise.resolve({blabla: '1'}); // creates a resolved promise
+        p.then(function (result) {
+            console.log(result); // --> {blabla: '1'};
+        });
+
+
+        var p2 = Promise.reject({error: 'error message'}); // creates a rejected promise
+        p2.catch(function (reason) {
+            console.log(reason); // --> {error: 'error message');
+        });
+
+
+    .. note:: Note that even if the promises are created already resolved or rejected, the `then` or `catch` handlers will still be called asynchronously.
+
+
+- based on an already asynchronous code
+    Suppose that in a function you must do a rpc, and when it is completed set the result on this.
+    The `this._rpc` is a function that returns a `Promise`.
+
+    .. code-block:: javascript
+
+        function callRpc() {
+            var self = this;
+            return this._rpc(...).then(function (result) {
+                self.myValueFromRpc = result;
+            });
+        }
+
+- for callback based function
+    Suppose that you were using a function `this.close` that takes as parameter a callback that is called when the closing is finished.
+    Now suppose that you are doing that in a method that must send a promise that is resolved when the closing is finished.
+
+    .. code-block:: javascript
+        :linenos:
+
+        function waitForClose() {
+            var self = this;
+            return new Promise (function(resolve, reject) {
+                self.close(resolve);
+            });
+        }
+
+    * line 2: we save the `this` into a variable so that in an inner function, we can access the scope of our component
+    * line 3: we create and return a new promise. The constructor of a promise takes a function as parameter. This function itself has 2 parameters that we called here `resolve` and `reject`
+        - `resolve` is a function that, when called, puts the promise in the resolved state.
+        - `reject` is a function that, when called, puts the promise in the rejected state. We do not use reject here and it can be omitted.
+    * line 4: we are calling the function close on our object. It takes a function as parameter (the callback) and it happens that resolve is already a function, so we can pass it directly. To be clearer, we could have written:
+
+    .. code-block:: javascript
+
+        return new Promise (function (resolve) {
+            self.close(function () {
+                resolve();
+            });
+        });
+
+
+
+- creating a promise generator (calling one promise after the other *in sequence* and waiting for the last one)
+    Suppose that you need to loop over an array, do an operation *in sequence* and resolve a promise when the last operation is done.
+
+    .. code-block:: javascript
+
+        function doStuffOnArray(arr) {
+            var done = Promise.resolve();
+            arr.forEach(function (item) {
+                done = done.then(function () {
+                    return item.doSomethingAsynchronous();
+                });
+            });
+            return done;
+        }
+
+    This way, the promise you return is effectively the last promise.
+- creating a promise, then resolving it outside the scope of its definition (anti-pattern)
+    .. note:: we do not recommend using this, but sometimes it is useful. Think carefully for alternatives first...
+
+    .. code-block:: javascript
+
+        ...
+        var resolver, rejecter;
+        var prom = new Promise(function (resolve, reject){
+            resolver = resolve;
+            rejecter = reject;
+        });
+        ...
+
+        resolver("done"); // will resolve the promise prom with the result "done"
+        rejecter("error"); // will reject the promise prom with the reason "error"
+
+Waiting for Promises
+--------------------
+- waiting for a number of Promises
+        if you have multiple promises that all need to be waited, you can convert them into a single promise that will be resolved when all the promises are resolved using Promise.all(arrayOfPromises).
+
+        .. code-block:: javascript
+
+                var prom1 = doSomethingThatReturnsAPromise();
+                var prom2 = Promise.resolve(true);
+                var constant = true;
+
+                var all = Promise.all([prom1, prom2, constant]); // all is a promise
+                // results is an array, the individual results correspond to the index of their
+                // promise as called in Promise.all()
+                all.then(function (results) {
+                    var prom1Result = results[0];
+                    var prom2Result = results[1];
+                    var constantResult = results[2];
+                });
+                return all;
+
+
+- waiting for a part of a promise chain, but not another part
+        If you have an asynchronous process that you want to wait to do something, but you also want to return to the caller before that something is done.
+
+        .. code-block:: javascript
+
+            function returnAsSoonAsAsyncProcessIsDone() {
+                var prom = AsyncProcess();
+                prom.then(function (resultOfAsyncProcess) {
+                        return doSomething();
+                });
+                /* returns prom which will only wait for AsyncProcess(),
+                   and when it will be resolved, the result will be the one of AsyncProcess */
+                return prom;
+            }
+
+Error handling
+--------------
+- in general in promises
+        The general idea is that a promise should not be rejected for control flow, but should only be rejected for errors.
+        When that is the case, you would have multiple resolutions of your promise with, for instance status codes that you would have to check in the `then` handlers and a single `catch` handler at the end of the promise chain.
+
+        .. code-block:: javascript
+
+            function a() {
+                x.y();  // <-- this is an error: x is undefined
+                return Promise.resolve(1);
+            }
+            function b() {
+               return Promise.reject(2);
+            }
+
+            a().catch(console.log);           // will log the error in a
+            a().then(b).catch(console.log);   // will log the error in a, the then is not executed
+            b().catch(console.log);           // will log the rejected reason of b (2)
+            Promise.resolve(1)
+                   .then(b)                   // the then is executed, it executes b
+                   .then(...)                 // this then is not executed
+                   .catch(console.log);       // will log the rejected reason of b (2)
+
+
+
+- in Odoo specifically
+        In Odoo, it happens that we use promise rejection for control flow, like in mutexes and other concurrency primitives defined in module `web.concurrency`
+        We also want to execute the catch for *business* reasons, but not when there is a coding error in the definition of the promise or of the handlers.
+        For this, we have introduced the concept of `guardedCatch`. It is called like `catch` but not when the rejected reason is an error
+
+        .. code-block:: javascript
+
+                function blabla() {
+                    if (someCondition) {
+                        return Promise.reject("someCondition is truthy");
+                    }
+                    return Promise.resolve();
+                }
+
+                // ...
+
+                var promise = blabla();
+                promise.then(function (result) { console.log("everything went fine"); })
+                // this will be called if blabla returns a rejected promise, but not if it has an error
+                promise.guardedCatch(function (reason) { console.log(reason); });
+
+                // ...
+
+                var anotherPromise =
+                        blabla().then(function () { console.log("everything went fine"); })
+                                // this will be called if blabla returns a rejected promise,
+                                // but not if it has an error
+                                .guardedCatch(console.log);
+
+
+        .. code-block:: javascript
+
+                var promiseWithError = Promise.resolve().then(function () {
+                    x.y();  // <-- this is an error: x is undefined
+                });
+                promiseWithError.guardedCatch(function (reason) {console.log(reason);}); // will not be called
+                promiseWithError.catch(function (reason) {console.log(reason);}); // will be called
+
+
+
+Testing asynchronous code
+--------------------------
+
+- using promises in tests
+        In the tests code, we support the latest version of Javascript, including primitives like `async` and `await`. This makes using and waiting for promises very easy.
+        Most helper methods also return a promise (either by being marked `async` or by returning a promise directly.
+
+        .. note:: Version 12 and 13 of Odoo supports IE11, for which it is not possible to polyfill async and await. We do not consider that it is important that the tests runs on these browser though.
+
+        .. code-block:: javascript
+
+                var testUtils = require('web.test_utils');
+                QUnit.test("My test", async function (assert) {
+                    // making the function async has 2 advantages:
+                    // 1) it always returns a promise so you don't need to define `var done = assert.async()`
+                    // 2) it allows you to use the `await`
+                    assert.expect(1);
+
+                    var form = await testUtils.createView({ ... });
+                    await testUtils.form.clickEdit(form);
+                    await testUtils.form.click('jquery selector');
+                    assert.containsOnce('jquery selector');
+                    form.destroy();
+                });
+
+                QUnit.test("My test - no async - no done", function (assert) {
+                    // this function is not async, but it returns a promise.
+                    // QUnit will wait for for this promise to be resolved.
+                    assert.expect(1);
+
+                    return testUtils.createView({ ... }).then(function (form) {
+                        return testUtils.form.clickEdit(form).then(function () {
+                            return testUtils.form.click('jquery selector').then(function () {
+                                assert.containsOnce('jquery selector');
+                                form.destroy();
+                            });
+                        });
+                    });
+                });
+
+
+                QUnit.test("My test - no async", function (assert) {
+                    // this function is not async and does not return a promise.
+                    // we have to use the done function to signal QUnit that the test is async and will be finished inside an async callback
+                    assert.expect(1);
+                    var done = assert.async();
+
+                    testUtils.createView({ ... }).then(function (form) {
+                        testUtils.form.clickEdit(form).then(function () {
+                            testUtils.form.click('jquery selector').then(function () {
+                            assert.containsOnce('jquery selector');
+                            form.destroy();
+                            done();
+                            });
+                        });
+                    });
+                });
+
+        as you can see, the nicer form is to use `async/await` as it is clearer and shorter to write.