Callbacks One approach to asynchronous programming is to make functions that perform a slow action take an extra argument, a callback function. The action is started, and when it finishes, the callback function is called with the result. As an example, the setTimeout function, available both in Node.js and in browsers, waits a given number of milliseconds (a second is a thousand milliseconds) and then calls a function. setTimeout(() => console.log("Tick"), 500);
Waiting is not generally a very important type of work, but it can be useful when doing something like updating an animation or checking whether something is taking longer than a given amount of time. Performing multiple asynchronous actions in a row using callbacks means that you have to keep passing new functions to handle the continuation of the computation after the actions. Most crow nest computers have a long-term data storage bulb, where pieces of information are etched into twigs so that they can be retrieved later. Etching, or finding a piece of data, takes a moment, so the interface to long-term storage is asynchronous and uses callback functions. Storage bulbs store pieces of JSON-encodable data under names. A crow might store information about the places where it’s hidden food under the name "food caches", which could hold an array of names that point at other pieces of data, describing the actual cache. To look up a food cache in the storage bulbs of the Big Oak nest, a crow could run code like this: import {bigOak} from "./crow-tech"; bigOak.readStorage("food caches", caches => { let firstCache = caches[0]; bigOak.readStorage(firstCache, info => { console.log(info); }); });
(All binding names and strings have been translated from crow language to English.) This style of programming is workable, but the indentation level increases
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