async await tutorial

async-examples.js

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()