then

then :: (Function fn, Any|Promise<Any> input) => Any|Promise<Any> output

description

A utility function which will call the provided function fn with the input as soon as the input's value is available: if the input is not a Promise, then(fn, input) is equivalent to fn(input). Otherwise, it's equivalent value.then(fn). It can be useful in a function composition chain if you're unsure as to whether the input value will be a Promise or not. It's mostly used internally in conductor.

If your value is a Promise, then's result will also be a Promise, so you'll need to use the await keyword or Promise.prototype.then. Since then's purpose is to execute code when you're unsure as if the input is actually a Promise, this can seem paradoxal. However, the await keyword will work seamlessly on Promise or non-Promise values.

example

synchronous example

import { then } from 'conductor'

const double = x => 2 * x
then(double, 2) // 4

Here, our input value (2) is not a Promise, so this code is equivalent to double(2).

asynchronous example

import { then } from 'conductor'

const double = x => 2 * x
then(double, Promise.resolve(2)) // Promise<pending>
await(double, Promise.resolve(2)) // 4

Here, our input value (Promise.resolve(2)) is a Promise, so then returns a Promise. If you try calling it without using the await keyword or Promise.prototype.then, the result will be a Promise.