nodejs-programming.md

Node.js Coding Practice

Q. What will happen when that code will be executed?

var EventEmitter = require("events");

var eventObj = new EventEmitter();

eventObj.on("event1", function () {
  console.log("Event1 fired!");
  process.nextTick(function () {
    eventObj.emit("event2");
  });
});

eventObj.on("event2", function () {
  console.log("Event2 fired!");
  process.nextTick(function () {
    eventObj.emit("event3");
  });
});

eventObj.on("event3", function () {
  console.log("Event3 fired!");
  process.nextTick(function () {
    eventObj.emit("event1");
  });
});

eventObj.emit("event1");

Answer

Event1 fired!
Event2 fired!
Event3 fired!
...
...
...
Event1 fired!
Event2 fired!
Event3 fired!

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Q. Rewrite the code sample without try/catch block

async function getData(req, res) {
  try {
    const a = await functionA();
    const b = await functionB();
    res.send("some result");
  } catch (error) {
    res.send(error.stack);
  }
}

Answer

async function getData() {
  const a = await functionA().catch((error) => console.log(error));
  const b = await functionB().catch((error) => console.log(error));
  if (a && b) {
    console.log("some result");
  }
}

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Q. Consider following code snippet

{
  console.time("loop");
  for (var i = 0; i < 1000000; i += 1) {
    // Do nothing
  }
  console.timeEnd("loop");
}

The time required to run this code in Google Chrome is considerably more than the time required to run it in Node.js Explain why this is so, even though both use the v8 JavaScript Engine.

Answer

Within a web browser such as Chrome, declaring the variable i outside of any function's scope makes it global and therefore binds it as a property of the window object. As a result, running this code in a web browser requires repeatedly resolving the property i within the heavily populated window namespace in each iteration of the for loop.

In Node.js, however, declaring any variable outside of any function's scope binds it only to the module's own scope (not the window object) which therefore makes it much easier and faster to resolve.

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Q. Rewrite promise-based Node.js applications to Async/Await

function asyncTask() {
  return functionA()
    .then((valueA) => functionB(valueA))
    .then((valueB) => functionC(valueB))
    .then((valueC) => functionD(valueC))
    .catch((err) => logger.error(err));
}

Answer

async function asyncTask() {
  try {
    const valueA = await functionA();
    const valueB = await functionB(valueA);
    const valueC = await functionC(valueB);
    return await functionD(valueC);
  } catch (err) {
    logger.error(err);
  }
}

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