Function apply with Promises

Question

I'm working on a promise-based project in Node.js using bluebird, and another in native promises by ES6. In both, I have a chain where I query a database in the following form:

some_function(/*...*/)

    .then(function () {
        return query("SELECT `whatever` FROM `wherever` ")
    })

    .then(/*...*/)

Note that query obviously returns a promise resolved to the query result. This repeats in several chains, and I'm looking for a way to clean up the unused function wrapper.

I'd naturally use Function.prototype.apply(), but in this case, when I try:

.then(query.apply(this, ["SELECT * FROM ... "]))
.then(function(rows){ /*...*/ })

The next function in the chain gets rows as undefined.

Thanks from ahead. Your help is appreciated.

Answer 1

You have to pass a function reference to .then() so your choices are as follows:

1. Use an inline anonymous function as you are.
2. Create your own utility function that returns another function (see example below)
3. Use .bind() to create another function.

The inline anonymous

some_function(/*...*/).then(function () {
    return query.apply("SELECT `whatever` FROM `wherever` ")
}).then(/*...*/)

Your own function wrapper

function queryWrap(q) {
    return function() {
        return query.apply(q);
    }
}

some_function(/*...*/)
  .then(queryWrap("SELECT `whatever` FROM `wherever` "))
  .then(/*...*/)

This wrapper could be useful if you could use it in multiple places. Probably not worth it for just one invocation.

Use .bind()

some_function(/*...*/)
  .then(query.apply.bind(query, "SELECT `whatever` FROM `wherever` "))
  .then(/*...*/)

Answer 2

In es6, arrow functions solve this best:

.then(() => query.apply("SELECT `whatever` FROM `wherever` "))
.then(rows => { /*...*/ })