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The bluebird library seems to automagically use Promise::then both as an equivalent of "map" and "flatMap" on the promise, eg see this example.
var Promise;
Promise = require('bluebird').Promise;
Promise.resolve(1).then(function(x) {
return Promise.resolve(x + 1);
}).then(function(x) {
return console.log(x); // => `2` (not a promise)
});
Promise.resolve(1).then(function(x) {
return x + 1;
}).then(function(x) {
return console.log(x); // => `2`
});
Promise.reject('hi').catch(function(x) {
return Promise.reject('hi2');
}).catch(function(x) {
return console.error(x); // => `hi2` (not a promise)
});
Is this a contract of the es6 Promise API? I see no mention of this flattening behavior here or here, for example.
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Apart from how the API is structured - promises have another major flaw: they treat unintentional native runtime exceptions and intentional rejected promises - which are two drastically different intentions - in the same "path".
If an error condition arises inside a promise, you “reject” the promise by calling the reject() function with an error. To handle a promise rejection, you pass a callback to the catch() function. This is a simple example, so catching the rejection is trivial.
Inside an async function, you can use the await keyword before a call to a function that returns a promise. This makes the code wait at that point until the promise is settled, at which point the fulfilled value of the promise is treated as a return value, or the rejected value is thrown.
As soon as the javascript Interpreter sees a promise declaration. It immediately executes its implementation synchronously. Even though it will get settled eventually.
Is this a contract of the es6 Promise API?
Yes, it is a contract established by Promises/A+, and has made its way from there into the ES6 specification. You will find some discussions here, here and here.
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