Q51-Q55

Q51. How JavaScript support asynchronous behavior?

Q52. What is a difference between callbacks and promises in JavaScript?
Q53. how to create a clone of object in JavaScript?
Q54. What are undeclared and undefined variables in JavaScript?
Q55. In how many ways you can clear Arrays in JavaScript?
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Q51. How JavaScript support asynchronous behavior?

Answer: 
It can be done through 

• Ajax (asynchronous JavaScript and xml) calls, 
• Callback functions,
• Javascript ES6 promises and 
• RXJS observables.

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Q52. What is a difference between callbacks and promises in JavaScript?

Answer : 

• Promises and callbacks are fundamentally same.
• Promises are introduced in ES6
• With promises we have much cleaner code with keyword .then() to handle returning result otherwise callback hell will seen in callback.
• With promises we have .catch() to catch error in much cleaner way.

Callback hell is an anti pattern  which means callback function inside a callback function. It would tough to maintain and readiliby is also not good with callbacks. 

http://qnimate.com/javascript-promise-vs-callback/
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Q53. how to create a clone of object in JavaScript?

Answer  53: 

Deep copy is also known as Cloning. 
There are two ways to copy objects in JavaScript. Both  ways to make a copy by value of objects as by default objects copy by reference. 
one is to make shallow copy and other is to make a deep copy.

Simple deep copy can be made by way JSON.parse(JSON.stringify(objA)).

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Q54. What are undeclared and undefined variables in JavaScript?

Answer 54:

1) In normal practice we first declare a variable and then define it.

2) var a, is declaring a variable. Here it is declared but undefined.

3) a = 30, giving some value to a variable is defining a variable.

(function(){

 var a; //declared but undefined

  console.log(a); //undefined

  var b;

  b = 10;

  // both declared and defined.

  c=12;

  // both declared and defined but declared globally.

})();

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Q55. In how many ways you can clear Arrays in JavaScript?

Answer 55:

1. assign array to empty array.
2. make array.length = 0.
3. using splice. use array.splice(0, array.length)
4. pop all elements using for loop.

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