Async and performance

Question 1

explain JS engine and the hosting environment

Answer 1

JS engine simply runs the code it is requested to by the hosting environment. The hosting environment could mostly be Browser or server environment like the node js

Question 2

explain event loop

Answer 2

It is basically similar to the event loop mechanism in window, where a loop continuously waits for some event to happen so that it can be processed.
This whole thing happens in the Engine.

Question 3

difference between async and parallel

Answer 3

Async is the difference between now and Later.Example ajax call, the point where the call is made is now and the callback which executes when the response comes back is later.
Parallel is about ability of multiple things running simultaneously

Question 4

explain concurrency

Answer 4

Concurrency is when two or more “processes” are executing simultaneously over the same period, regardless of whether their individual constituent operations happen in parallel (at the same instant on separate processors or cores) or not.

Question 5

What is Thenable object?

Answer 5

Any obect that has which has a then(..) method on it. It is assumed that any such value is a Promise-conforming thenable.

Question 6

Are promises called asynchronously?

Answer 6

That is, when you call then(..) on a Promise, even if that Promise was already resolved, the callback you provide to then(..) will always be called asynchronously

Question 7

What does Promise.resolve() do?

Answer 7

Promise.resolve(..) will accept any thenable, and will unwrap it to its non-thenable value. But you get back from Promise.resolve(..) a real, genuine Promise in its place, one that you can trust. If what you passed in is already a genuine Promise, you just get it right back, so there’s no downside at all to filtering through Promise.resolve(..) to gain trust.

Question 8

When should Promise.resolve be called?

Answer 8

It should be called when we call a method, returning promise or a “then able” object but we don’t trust that its return value to be well behaving promise.

Question 9

what happens when a reject handler is not supplied for. Promise handler?

Answer 9

an assumed rejection handler is substituted:the assumed rejection handler simply rethrows the error, which ends up forcing the chained promise to reject with the same error reason. In essence, this allows the error to continue propagating along a Promise chain until an explicitly defined rejection handler is encountered.

Question 10

what is the output?
function foo() {
	setTimeout( function(){
		baz.bar();
	}, 100 );
}

try {
foo();

}
catch (err) {
console.log(“Exception occured”)
}

Answer 10

The error never gets caught because try/catch in sync and the exception happens in the setTimeout which is async

Question 11

what is the output?
var p = Promise.reject( "Oops" );

p.then(
	function fulfilled(){
	},
	function rejected(err){
		console.log( err ); 
	}
);

Answer 11

Oops

Question 12

what is the output?

var p = Promise.resolve( 42 );

p.then(
	function fulfilled(msg){

		console.log( msg.toLowerCase() );
	},
	function rejected(err){
		console.log("error occured")
	}
);

Answer 12

Exception will be thrown.

The rejected handler won’t get called since, the error handler is for the promise p

Question 13

What happens when an error is thrown inside the done handler?

var p = Promise.resolve( 42 );

p.then(
	function fulfilled(msg){
		// numbers don't have string functions,
		// so will throw an error
		console.log( msg.toLowerCase() );
	}
)
.done( null, handleErrors );

Answer 13

It will be thrown as a global error in

Question 14

What does Promise.all([ .. ]) do?

Answer 14

Takes in an array of promises, executes them on parallel and calls the success/reject handlers appropriately

Question 15

what does Promise.race([ .. ]) do?

Answer 15

Takes in an array of promises and resolves as soon as the first one completes

Question 16

what is the output?
var p1 = Promise.resolve( 42 );
var p2 = Promise.resolve( "Hello World" );
var p3 = Promise.reject( "Oops" );

Promise.race( [p1,p2,p3] )
.then( function(msg){
console.log( msg );
} );

Promise.all( [p1,p2,p3] )
.catch( function(err){
console.error( err );
} );

Promise.all( [p1,p2] )
.then( function(msgs){
console.log( msgs );
} );

Answer 16

42
“Oops”
[42,”Hello World”]

Question 17

var p = foo( 42 )
.then( STEP2 )
.then( STEP3 );

what happens when an error gets thrown in one of the steps

Answer 17

since none of the promises have an error handler it will propagate indefinitely.

p.catch(…)
will handle the error

Question 18

How to pause a function execution at some point and resume later

Answer 18

Use of “yield” keyword

Question 19

what is the output?
function *foo() {
	var x = yield 2;
	z++;
	var y = yield (x * z);
	console.log( x, y, z );
}

var z = 1;

var it1 = foo();
var it2 = foo();

var val1 = it1.next().value;			
var val2 = it2.next().value;			

val1 = it1.next( val2 * 10 ).value;		
val2 = it2.next( val1 * 5 ).value;		

it1. next( val2 / 2 );
it2. next( val1 / 4 );

Answer 19

20, 300, 3

200 10 3

Question 20

How to make an object iterable ?

Answer 20

return {

	[Symbol.iterator]: function(){ return this; }, }

Question 21

How to get all the properties of an Object?

Answer 21

var obj = new SomeObject()
var props = Object.keys(obj)
//This method doesnt return the properties from teh prototype