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Async function handling examples
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| // https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Introducing | |
| var inputString = 'Hello World'; | |
| function callbackStyleFunction (input, cb) { | |
| //if (someErrorCondition) return cb('something bad happened'); | |
| cb(null, input); | |
| }; | |
| const promiseStyleFunction = function (input) { | |
| return new Promise(function (resolve, reject) { | |
| //if (someErrorCondition) return reject('something bad happened') | |
| resolve(input) | |
| }) | |
| } | |
| // Example 1 - callback | |
| callbackStyleFunction(inputString, function (errorFromCallback, resultOfCallback) { | |
| console.log('callback finished'); | |
| console.log(errorFromCallback); | |
| console.log(resultOfCallback+' from callback'); | |
| // Example 2 - chained promises (this will fire pretty much at the same time as example 3) | |
| const { promisify } = require('util') | |
| const returnInput = promisify(callbackStyleFunction) | |
| returnInput(inputString) | |
| .then(function (resultOfCallbackAsPromise) { | |
| console.log('do something else after') | |
| console.log(resultOfCallbackAsPromise+' from 1st chained function') | |
| return resultOfCallbackAsPromise | |
| }) | |
| .then(async function (resultOfCallbackAsPromise) { | |
| console.log('finally do something here') | |
| console.log(resultOfCallbackAsPromise+' from 2nd chained function') | |
| const resultOfPromise = await promiseStyleFunction(inputString) | |
| console.log(resultOfPromise+' from promise awaited in the async function that is the 2nd chained function') | |
| }) | |
| .catch(function (errorFromCallbackAsPromise) { | |
| console.log('whoops, chain of promises broke') | |
| console.log(errorFromCallbackAsPromise) | |
| }) | |
| // Example 3 - async / await the callback function (this will fire pretty much the same time as example 2, just after it in reality) | |
| ;/*here be dragons*/(async function () { | |
| try { | |
| var resultOfCallbackAsPromise = await returnInput(inputString) | |
| console.log('a bit easier') | |
| console.log(resultOfCallbackAsPromise+' from callback converted to promise and awaited in an async IIFE') | |
| const resultOfPromise = await promiseStyleFunction(inputString) | |
| console.log(resultOfPromise+' from promise awaited in an async IIFE') | |
| } catch (errorFromCallbackAsPromise) { | |
| console.log('whoops, async did not await the callback function') | |
| console.log(errorFromCallbackAsPromise) | |
| } | |
| })() | |
| }) | |
| // Example 4 - async / await the promise function (this will fire pretty much the same time as example 1, just after it in reality) | |
| async function showTheInput () { | |
| try { | |
| const resultOfPromise = await promiseStyleFunction(inputString) | |
| console.log('a bit more easier') | |
| console.log(resultOfPromise+' from promise awaited in an async function') | |
| } catch (errorFromPromise) { | |
| console.log('whoops, async did not await the promise function') | |
| console.log(errorFromPromise) | |
| } | |
| } | |
| showTheInput() |
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