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July 23, 2023 16:42
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async await tutorial
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| function log(...args) { | |
| console.log(new Date(), ...args); | |
| } | |
| // Compute intensive synchronous task | |
| function doMatrixMult(n) { | |
| let out = 1; | |
| for (let index = 0; index < n; index++) { | |
| out = out + Math.sin(index); | |
| } | |
| log(`Result of computing for ${n} is `, out); | |
| return out; | |
| } | |
| // call back style asynchronous task | |
| function asyncTask1({ id, wait }, cb) { | |
| log("Task with id = ", id, " is starting"); | |
| setTimeout(function () { | |
| log(`task with id ${id} is done`); | |
| if (id % 2 === 0) { | |
| cb(null, `taks ${id} done`); | |
| } else { | |
| cb(new Error(`task ${id} failed`)); | |
| } | |
| }, wait * 1000); | |
| } | |
| // call back style asynchronous task converted to promise. | |
| // It is equivalent to an async function | |
| function asyncTaskPromise(arg1) { | |
| return new Promise(function (resolve, reject) { | |
| asyncTask1(arg1, function (err, res) { | |
| if (err) { | |
| return reject(err); | |
| } | |
| return resolve(res); | |
| }); | |
| }); | |
| } | |
| // An async function ( or promise ) is converted back to callback style async function | |
| function asyncTaskPromiseCbStyle(arg1, cb) { | |
| asyncTaskPromise(arg1) | |
| .then(function (res) { | |
| cb(null, res); | |
| }) | |
| .catch(function (err) { | |
| cb(err); | |
| }); | |
| } | |
| /* Example cases */ | |
| function promiseChainExample() { | |
| // Example of promise chain. | |
| asyncTaskPromise({ id: 8, wait: 2 }) | |
| .then(function () { | |
| return asyncTaskPromise({ id: 5, wait: 1 }).catch(function (err) { | |
| log("Promise 5 failed but ook"); | |
| }); | |
| }) | |
| .then(function () { | |
| asyncTaskPromise({ id: 4, wait: 5 }) | |
| .then(function () { | |
| doMatrixMult(110000); | |
| }) | |
| .catch((err) => { | |
| console.log("Error at promise 4", err); | |
| }); | |
| return asyncTaskPromise({ id: 6, wait: 2 }); | |
| }) | |
| .then(function () { | |
| doMatrixMult(7000); | |
| }) | |
| .catch(function (err) { | |
| log("Error occured", err); | |
| log("All tasks completed"); | |
| }); | |
| } | |
| // asynchronous function | |
| async function async_sum(a, b) { | |
| return a + b; | |
| } | |
| // equivalent of the above function. | |
| function promise_sum(a, b) { | |
| return new Promise(function (resolve, reject) { | |
| resolve(a + b); | |
| }); | |
| } | |
| // normal function | |
| function sum(a, b) { | |
| return a + b; | |
| } | |
| async function asyncAwaitExample() { | |
| let x = sum(3, 2); | |
| log("a+b", x + 1); | |
| x = async_sum(3, 2); | |
| log("a+b", x + 1); | |
| x = promise_sum(3, 2); | |
| log("a+b", x + 1); | |
| let y = await promise_sum(8, 9); | |
| console.log("y is ", y); | |
| try { | |
| await asyncTaskPromise({ id: 2, wait: 2.0 }); | |
| await asyncTaskPromise({ id: 7, wait: 0.1 }); | |
| await asyncTaskPromise({ id: 4, wait: 1.5 }); | |
| await asyncTaskPromise({ id: 8, wait: 0.5 }); | |
| await asyncTaskPromise({ id: 10, wait: 0.2 }); | |
| } catch (error) { | |
| log("error at ", error); | |
| } | |
| } | |
| function parallelPromiseExample() { | |
| Promise.all([ | |
| asyncTaskPromise({ id: 2, wait: 2.0 }), | |
| asyncTaskPromise({ id: 4, wait: 1.5 }), | |
| asyncTaskPromise({ id: 8, wait: 0.5 }), | |
| asyncTaskPromise({ id: 10, wait: 0.2 }), | |
| asyncTaskPromise({ id: 7, wait: 0.1 }), | |
| ]) | |
| .then(function (items) { | |
| log("All promises completed", items); | |
| }) | |
| .catch(function (err) { | |
| log("Something went wrong", err); | |
| }); | |
| } | |
| function callBackSyleExample() { | |
| try { | |
| asyncTask1({ id: 8, wait: 2 }, function (err) { | |
| if (err) { | |
| throw err; | |
| } | |
| asyncTask1({ id: 2, wait: 1 }, function (err) { | |
| if (err) { | |
| throw err; | |
| } | |
| asyncTask1({ id: 4, wait: 5 }, function (err) { | |
| if (err) { | |
| throw err; | |
| } | |
| doMatrixMult(11000); | |
| }); | |
| asyncTask1({ id: 3, wait: 2 }, function (err) { | |
| if (err) { | |
| throw err; | |
| } | |
| doMatrixMult(7000); | |
| }); | |
| }); | |
| }); | |
| } catch (err) { | |
| log("Error occured", err); | |
| log("All tasks completed"); | |
| } | |
| } | |
| // callBackSyleExample(); | |
| // promiseChainExample(); | |
| // parallelPromiseExample(); | |
| asyncAwaitExample() |
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