Js-Interview-Questions/docs/05-async-event-loop.md at master · BilgeGates/Js-Interview-Questions

No.	Questions

| 191 | What is the purpose of using object is method | | 192 | How do you copy properties from one object to other | | 193 | What are the applications of assign method | | 194 | What is a proxy object | | 195 | What is the purpose of seal method | | 196 | What are the applications of seal method | | 197 | What are the differences between freeze and seal methods | | 198 | How do you determine if an object is sealed or not | | 199 | How do you get enumerable key and value pairs | | 200 | What is the main difference between Object.values and Object.entries method | | 201 | How can you get the list of keys of any object | | 202 | How do you create an object with prototype | | 203 | What is a WeakSet | | 204 | What are the differences between WeakSet and Set | | 205 | List down the collection of methods available on WeakSet | | 206 | What is a WeakMap | | 207 | What are the differences between WeakMap and Map | | 208 | List down the collection of methods available on WeakMap | | 209 | What is the purpose of uneval | | 210 | How do you encode an URL | | 211 | How do you decode an URL | | 212 | How do you print the contents of web page | | 213 | What is the difference between uneval and eval | | 214 | What is an anonymous function | | 215 | What is the precedence order between local and global variables | | 216 | What are javascript accessors | | 217 | How do you define property on Object constructor | | 218 | What is the difference between get and defineProperty | | 219 | What are the advantages of Getters and Setters | | 220 | Can I add getters and setters using defineProperty method | | 221 | What is the purpose of switch-case | | 222 | What are the conventions to be followed for the usage of switch case | | 223 | What are primitive data types | | 224 | What are the different ways to access object properties | | 225 | What are the function parameter rules | | 226 | What is an error object | | 227 | When you get a syntax error | | 228 | What are the different error names from error object | | 229 | What are the various statements in error handling | | 230 | What are the two types of loops in javascript | | 231 | What is nodejs | | 232 | What is an Intl object | | 233 | How do you perform language specific date and time formatting | | 234 | What is an Iterator | | 235 | How does synchronous iteration works | | 236 | What is an event loop | | 237 | What is call stack | | 238 | What is an event queue | | 239 | What is a decorator | | 240 | What are the properties of Intl object |

What is the purpose of using object is method

Some of the applications of Object's is method are follows,
1. It is used for comparison of two strings.
2. It is used for comparison of two numbers.
3. It is used for comparing the polarity of two numbers.
4. It is used for comparison of two objects.
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How do you copy properties from one object to other

You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the source objects. The syntax would be as below,
```
Object.assign(target, ...sources);
```
Let's take example with one source and one target object,
```
const target = { a: 1, b: 2 };
const source = { b: 3, c: 4 };

const returnedTarget = Object.assign(target, source);

console.log(target); // { a: 1, b: 3, c: 4 }

console.log(returnedTarget); // { a: 1, b: 3, c: 4 }
```
As observed in the above code, there is a common property(b) from source to target so it's value has been overwritten.

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What are the applications of assign method

Below are the some of main applications of Object.assign() method,
1. It is used for cloning an object.
2. It is used to merge objects with the same properties.
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What is a proxy object

The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows,
```
var p = new Proxy(target, handler);
```
Let's take an example of proxy object,
```
var handler = {
  get: function (obj, prop) {
    return prop in obj ? obj[prop] : 100;
  },
};

var p = new Proxy({}, handler);
p.a = 10;
p.b = null;

console.log(p.a, p.b); // 10, null
console.log("c" in p, p.c); // false, 100
```
In the above code, it uses get handler which define the behavior of the proxy when an operation is performed on it

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What is the purpose of seal method

The Object.seal() method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let's see the below example to understand more about seal() method
```
const object = {
  property: "Welcome JS world",
};
Object.seal(object);
object.property = "Welcome to object world";
console.log(Object.isSealed(object)); // true
delete object.property; // You cannot delete when sealed
console.log(object.property); //Welcome to object world
```
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What are the applications of seal method

Below are the main applications of Object.seal() method,
1. It is used for sealing objects and arrays.
2. It is used to make an object immutable.
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What are the differences between freeze and seal methods

If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.

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How do you determine if an object is sealed or not

The Object.isSealed() method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true
1. If it is not extensible.
2. If all of its properties are non-configurable.
3. If it is not removable (but not necessarily non-writable). Let's see it in the action
```
const object = {
  property: "Hello, Good morning",
};

Object.seal(object); // Using seal() method to seal the object

console.log(Object.isSealed(object)); // checking whether the object is sealed or not
```
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How do you get enumerable key and value pairs

The Object.entries() method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. Let's see the functionality of object.entries() method in an example,
```
const object = {
  a: "Good morning",
  b: 100,
};

for (let [key, value] of Object.entries(object)) {
  console.log(`${key}: ${value}`); // a: 'Good morning'
  // b: 100
}
```
Note: The order is not guaranteed as object defined.

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What is the main difference between Object.values and Object.entries method

The Object.values() method's behavior is similar to Object.entries() method but it returns an array of values instead [key,value] pairs.
```
const object = {
  a: "Good morning",
  b: 100,
};

for (let value of Object.values(object)) {
  console.log(`${value}`); // 'Good morning'
  100;
}
```
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How can you get the list of keys of any object

You can use the Object.keys() method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,
```
const user = {
  name: "John",
  gender: "male",
  age: 40,
};

console.log(Object.keys(user)); //['name', 'gender', 'age']
```
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How do you create an object with prototype

The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.

const user = {
  name: "John",
  printInfo: function () {
    console.log(`My name is ${this.name}.`);
  },
};

const admin = Object.create(user);

admin.name = "Nick"; // Remember that "name" is a property set on "admin" but not on "user" object

admin.printInfo(); // My name is Nick

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What is a WeakSet

WeakSet is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,

new WeakSet([iterable]);

Let's see the below example to explain it's behavior,

var ws = new WeakSet();
var user = {};
ws.add(user);
ws.has(user); // true
ws.delete(user); // removes user from the set
ws.has(user); // false, user has been removed

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What are the differences between WeakSet and Set

The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it. Other differences are,
1. Sets can store any value Whereas WeakSets can store only collections of objects
2. WeakSet does not have size property unlike Set
3. WeakSet does not have methods such as clear, keys, values, entries, forEach.
4. WeakSet is not iterable.
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List down the collection of methods available on WeakSet

Below are the list of methods available on WeakSet,
1. add(value): A new object is appended with the given value to the weakset
2. delete(value): Deletes the value from the WeakSet collection.
3. has(value): It returns true if the value is present in the WeakSet Collection, otherwise it returns false.
Let's see the functionality of all the above methods in an example,
```
var weakSetObject = new WeakSet();
var firstObject = {};
var secondObject = {};
// add(value)
weakSetObject.add(firstObject);
weakSetObject.add(secondObject);
console.log(weakSetObject.has(firstObject)); //true
weakSetObject.delete(secondObject);
```
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What is a WeakMap

The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below,
```
new WeakMap([iterable]);
```
Let's see the below example to explain it's behavior,
```
var ws = new WeakMap();
var user = {};
ws.set(user);
ws.has(user); // true
ws.delete(user); // removes user from the map
ws.has(user); // false, user has been removed
```
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What are the differences between WeakMap and Map

The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it. Other differences are,
1. Maps can store any key type Whereas WeakMaps can store only collections of key objects
2. WeakMap does not have size property unlike Map
3. WeakMap does not have methods such as clear, keys, values, entries, forEach.
4. WeakMap is not iterable.
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List down the collection of methods available on WeakMap

Below are the list of methods available on WeakMap,
1. set(key, value): Sets the value for the key in the WeakMap object. Returns the WeakMap object.
2. delete(key): Removes any value associated to the key.
3. has(key): Returns a Boolean asserting whether a value has been associated to the key in the WeakMap object or not.
4. get(key): Returns the value associated to the key, or undefined if there is none. Let's see the functionality of all the above methods in an example,
```
var weakMapObject = new WeakMap();
var firstObject = {};
var secondObject = {};
// set(key, value)
weakMapObject.set(firstObject, "John");
weakMapObject.set(secondObject, 100);
console.log(weakMapObject.has(firstObject)); //true
console.log(weakMapObject.get(firstObject)); // John
weakMapObject.delete(secondObject);
```
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What is the purpose of uneval

The uneval() is an inbuilt function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let's see the below example to know more about it's functionality,
```
var a = 1;
uneval(a); // returns a String containing 1
uneval(function user() {}); // returns "(function user(){})"
```
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How do you encode an URL

The encodeURI() function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.

var uri = "https://mozilla.org/?x=шеллы";
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B

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How do you decode an URL

The decodeURI() function is used to decode a Uniform Resource Identifier (URI) previously created by encodeURI().

var uri = "https://mozilla.org/?x=шеллы";
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
try {
  console.log(decodeURI(encoded)); // "https://mozilla.org/?x=шеллы"
} catch (e) {
  // catches a malformed URI
  console.error(e);
}

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How do you print the contents of web page

The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example,
```
<input type="button" value="Print" onclick="window.print()" />
```
Note: In most browsers, it will block while the print dialog is open.

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What is the difference between uneval and eval

The uneval function returns the source of a given object; whereas the eval function does the opposite, by evaluating that source code in a different memory area. Let's see an example to clarify the difference,
```
var msg = uneval(function greeting() {
  return "Hello, Good morning";
});
var greeting = eval(msg);
greeting(); // returns "Hello, Good morning"
```
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What is an anonymous function

An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,

function (optionalParameters) {
  //do something
}

const myFunction = function(){ //Anonymous function assigned to a variable
  //do something
};

[1, 2, 3].map(function(element){ //Anonymous function used as a callback function
  //do something
});

Let's see the above anonymous function in an example,

var x = function (a, b) {
  return a * b;
};
var z = x(5, 10);
console.log(z); // 50

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What is the precedence order between local and global variables

A local variable takes precedence over a global variable with the same name. Let's see this behavior in an example.
```
var msg = "Good morning";
function greeting() {
  msg = "Good Evening";
  console.log(msg); // Good Evening
}
greeting();
```
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What are javascript accessors

ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the get keyword whereas Setters uses the set keyword.

var user = {
  firstName: "John",
  lastName : "Abraham",
  language : "en",
  get lang() {
    return this.language;
  },
  set lang(lang) {
  this.language = lang;
  }
};
console.log(user.lang); // getter access lang as en
user.lang = 'fr';
console.log(user.lang); // setter used to set lang as fr

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How do you define property on Object constructor

The Object.defineProperty() static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let's see an example to know how to define property,
```
const newObject = {};

Object.defineProperty(newObject, "newProperty", {
  value: 100,
  writable: false,
});

console.log(newObject.newProperty); // 100

newObject.newProperty = 200; // It throws an error in strict mode due to writable setting
```
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What is the difference between get and defineProperty

Both have similar results until unless you use classes. If you use get the property will be defined on the prototype of the object whereas using Object.defineProperty() the property will be defined on the instance it is applied to.

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What are the advantages of Getters and Setters

Below are the list of benefits of Getters and Setters,
1. They provide simpler syntax
2. They are used for defining computed properties, or accessors in JS.
3. Useful to provide equivalence relation between properties and methods
4. They can provide better data quality
5. Useful for doing things behind the scenes with the encapsulated logic.
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Can I add getters and setters using defineProperty method

Yes, You can use the Object.defineProperty() method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,

var obj = { counter: 0 };

// Define getters
Object.defineProperty(obj, "increment", {
  get: function () {
    this.counter++;
  },
});
Object.defineProperty(obj, "decrement", {
  get: function () {
    this.counter--;
  },
});

// Define setters
Object.defineProperty(obj, "add", {
  set: function (value) {
    this.counter += value;
  },
});
Object.defineProperty(obj, "subtract", {
  set: function (value) {
    this.counter -= value;
  },
});

obj.add = 10;
obj.subtract = 5;
console.log(obj.increment); //6
console.log(obj.decrement); //5

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What is the purpose of switch-case

The switch case statement in JavaScript is used for decision making purposes. In a few cases, using the switch case statement is going to be more convenient than if-else statements. The syntax would be as below,
```
switch (expression)
{
    case value1:
        statement1;
        break;
    case value2:
        statement2;
        break;
    .
    .
    case valueN:
        statementN;
        break;
    default:
        statementDefault;
}
```
The above multi-way branch statement provides an easy way to dispatch execution to different parts of code based on the value of the expression.

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What are the conventions to be followed for the usage of switch case

Below are the list of conventions should be taken care,
1. The expression can be of type either number or string.
2. Duplicate values are not allowed for the expression.
3. The default statement is optional. If the expression passed to switch does not match with any case value then the statement within default case will be executed.
4. The break statement is used inside the switch to terminate a statement sequence.
5. The break statement is optional. But if it is omitted, the execution will continue on into the next case.
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What are primitive data types

A primitive data type is data that has a primitive value (which has no properties or methods). There are 7 types of primitive data types.
1. string
2. number
3. boolean
4. null
5. undefined
6. bigint
7. symbol
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What are the different ways to access object properties

There are 3 possible ways for accessing the property of an object.
1. Dot notation: It uses dot for accessing the properties
```
objectName.property;
```
1. Square brackets notation: It uses square brackets for property access
```
objectName["property"];
```
1. Expression notation: It uses expression in the square brackets
```
objectName[expression];
```
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What are the function parameter rules

JavaScript functions follow below rules for parameters,
1. The function definitions do not specify data types for parameters.
2. Do not perform type checking on the passed arguments.
3. Do not check the number of arguments received. i.e, The below function follows the above rules,
```
function functionName(parameter1, parameter2, parameter3) {
  console.log(parameter1); // 1
}
functionName(1);
```
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What is an error object

An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,
```
try {
  greeting("Welcome");
} catch (err) {
  console.log(err.name + "<br>" + err.message);
}
```
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When you get a syntax error

A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error
```
try {
  eval("greeting('welcome)"); // Missing ' will produce an error
} catch (err) {
  console.log(err.name);
}
```
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What are the different error names from error object

There are 6 different types of error names returned from error object,

Error Name	Description
EvalError	An error has occurred in the eval() function
RangeError	An error has occurred with a number "out of range"
ReferenceError	An error due to an illegal reference
SyntaxError	An error due to a syntax error
TypeError	An error due to a type error
URIError	An error due to encodeURI()

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What are the various statements in error handling

Below are the list of statements used in an error handling,
1. try: This statement is used to test a block of code for errors
2. catch: This statement is used to handle the error
3. throw: This statement is used to create custom errors.
4. finally: This statement is used to execute code after try and catch regardless of the result.
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What are the two types of loops in javascript
1. Entry Controlled loops: In this kind of loop type, the test condition is tested before entering the loop body. For example, For Loop and While Loop comes under this category.
2. Exit Controlled Loops: In this kind of loop type, the test condition is tested or evaluated at the end of the loop body. i.e, the loop body will execute at least once irrespective of test condition true or false. For example, do-while loop comes under this category.
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What is nodejs

Node.js is a server-side platform built on Chrome's JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google's V8 JavaScript engine and libuv library.

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What is an Intl object

The Intl object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.

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How do you perform language specific date and time formatting

You can use the Intl.DateTimeFormat object which is a constructor for objects that enable language-sensitive date and time formatting. Let's see this behavior with an example,
```
var date = new Date(Date.UTC(2019, 07, 07, 3, 0, 0));
console.log(new Intl.DateTimeFormat("en-GB").format(date)); // 07/08/2019
console.log(new Intl.DateTimeFormat("en-AU").format(date)); // 07/08/2019
```
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What is an Iterator

An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a next() method which returns an object with two properties: value (the next value in the sequence) and done (which is true if the last value in the sequence has been consumed).

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How does synchronous iteration works

Synchronous iteration was introduced in ES6 and it works with below set of components,

Iterable: It is an object which can be iterated over via a method whose key is Symbol.iterator. Iterator: It is an object returned by invoking [Symbol.iterator]() on an iterable. This iterator object wraps each iterated element in an object and returns it via next() method one by one. IteratorResult: It is an object returned by next() method. The object contains two properties; the value property contains an iterated element and the done property determines whether the element is the last element or not.

Let's demonstrate synchronous iteration with an array as below,
```
const iterable = ["one", "two", "three"];
const iterator = iterable[Symbol.iterator]();
console.log(iterator.next()); // { value: 'one', done: false }
console.log(iterator.next()); // { value: 'two', done: false }
console.log(iterator.next()); // { value: 'three', done: false }
console.log(iterator.next()); // { value: 'undefined, done: true }
```
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What is an event loop

The event loop is a process that continuously monitors both the call stack and the event queue and checks whether or not the call stack is empty. If the call stack is empty and there are pending events in the event queue, the event loop dequeues the event from the event queue and pushes it to the call stack. The call stack executes the event, and any additional events generated during the execution are added to the end of the event queue.

Note: The event loop allows Node.js to perform non-blocking I/O operations, even though JavaScript is single-threaded, by offloading operations to the system kernel whenever possible. Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background.

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What is call stack

Call Stack is a data structure for javascript interpreters to keep track of function calls(creates execution context) in the program. It has two major actions,
1. Whenever you call a function for its execution, you are pushing it to the stack.
2. Whenever the execution is completed, the function is popped out of the stack.
Let's take an example and it's state representation in a diagram format
```
function hungry() {
  eatFruits();
}
function eatFruits() {
  return "I'm eating fruits";
}

// Invoke the `hungry` function
hungry();
```
The above code processed in a call stack as below,
1. Add the hungry() function to the call stack list and execute the code.
2. Add the eatFruits() function to the call stack list and execute the code.
3. Delete the eatFruits() function from our call stack list.
4. Delete the hungry() function from the call stack list since there are no items anymore.
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What is an event queue

The event queue follows the queue data structure. It stores async callbacks to be added to the call stack. It is also known as the Callback Queue or Macrotask Queue.

Whenever the call stack receives an async function, it is moved into the Web API. Based on the function, Web API executes it and awaits the result. Once it is finished, it moves the callback into the event queue (the callback of the promise is moved into the microtask queue).

The event queue constantly checks whether or not the call stack is empty. Once the call stack is empty and there is a callback in the event queue, the event queue moves the callback into the call stack. If there is a callback in the microtask queue as well, it is moved first. The microtask queue has a higher priority than the event queue.

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What is a decorator

A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let's define admin decorator for user class at design time,
```
function admin(isAdmin) {
   return function(target) {
       target.isAdmin = isAdmin;
   }
}

@admin(true)
class User() {
}
console.log(User.isAdmin); //true

 @admin(false)
 class User() {
 }
 console.log(User.isAdmin); //false
```
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What are the properties of Intl object

Below are the list of properties available on Intl object,
1. Collator: These are the objects that enable language-sensitive string comparison.
2. DateTimeFormat: These are the objects that enable language-sensitive date and time formatting.
3. ListFormat: These are the objects that enable language-sensitive list formatting.
4. NumberFormat: Objects that enable language-sensitive number formatting.
5. PluralRules: Objects that enable plural-sensitive formatting and language-specific rules for plurals.
6. RelativeTimeFormat: Objects that enable language-sensitive relative time formatting.
⬆ Back to Top

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Table of Contents

What is the purpose of using object is method

How do you copy properties from one object to other

What are the applications of assign method

What is a proxy object

What is the purpose of seal method

What are the applications of seal method

What are the differences between freeze and seal methods

How do you determine if an object is sealed or not

How do you get enumerable key and value pairs

What is the main difference between Object.values and Object.entries method

How can you get the list of keys of any object

How do you create an object with prototype

What is a WeakSet

What are the differences between WeakSet and Set

List down the collection of methods available on WeakSet

What is a WeakMap

What are the differences between WeakMap and Map

List down the collection of methods available on WeakMap

What is the purpose of uneval

How do you encode an URL

How do you decode an URL

How do you print the contents of web page

What is the difference between uneval and eval

What is an anonymous function

What is the precedence order between local and global variables

What are javascript accessors

How do you define property on Object constructor

What is the difference between get and defineProperty

What are the advantages of Getters and Setters

Can I add getters and setters using defineProperty method

What is the purpose of switch-case

What are the conventions to be followed for the usage of switch case

What are primitive data types

What are the different ways to access object properties

What are the function parameter rules

What is an error object

When you get a syntax error

What are the different error names from error object

What are the various statements in error handling

What are the two types of loops in javascript

What is nodejs

What is an Intl object

How do you perform language specific date and time formatting

What is an Iterator

How does synchronous iteration works

What is an event loop

What is call stack

What is an event queue

What is a decorator

What are the properties of Intl object