Using closures for maintaining the scope of variables (in our case, the success callback function parameters) is the natural JavaScript solution. With promises, we can arbitrarily nest and flatten .then() callbacks - they are semantically equivalent, except for the scope of the inner one.
function getExample() {
return promiseA(…).then(function(resultA) {
// some processing
return promiseB(…).then(function(resultB) {
// more processing
return // something using both resultA and resultB;
});
});
}
Of course, this is building an indentation pyramid. If indentation is getting too large, you still can apply the old tools to counter the pyramid of doom: modularize, use extra named functions, and flatten the promise chain as soon as you don't need a variable any more.
In theory, you can always avoid more than two levels of nesting (by making all closures explicit), in practise use as many as are reasonable.
function getExample() {
// preprocessing
return promiseA(…).then(makeAhandler(…));
}
function makeAhandler(…)
return function(resultA) {
// some processing
return promiseB(…).then(makeBhandler(resultA, …));
};
}
function makeBhandler(resultA, …) {
return function(resultB) {
// more processing
return // anything that uses the variables in scope
};
}
You can also use helper functions for this kind of partial application, like _.partial from Underscore/lodash or the native .bind() method, to further decrease indentation:
function getExample() {
// preprocessing
return promiseA(…).then(handlerA);
}
function handlerA(resultA) {
// some processing
return promiseB(…).then(handlerB.bind(null, resultA));
}
function handlerB(resultA, resultB) {
// more processing
return // anything that uses resultA and resultB
}