JS Object, This, & JSON

Question 1

Can a property be deleted from an object?

Answer 1

There isn’t any method in an Object itself to delete its own properties (such as Map.prototype.delete()). To do so, one must use the delete operator.

(Map is preferable if deletion is necessary)

Question 2

Object.assign()

Answer 2

Copies the values of all enumerable own properties from one or more source objects to a target object.

Question 3

Object.create()

Answer 3

Creates a new object with the specified prototype object and properties.

Question 4

Object.defineProperty()

Object.defineProperties()

Answer 4

Adds the named properties described by the given descriptors to an object.

Question 5

Object.entries()

Object.fromEntries()

Answer 5

Returns an array containing all of the [key, value] pairs of a given object’s own enumerable string properties.

and the reverse, returns a new object from an iterable of [key, value] pairs.

Question 6

Object.freeze()

Object.isFrozen()

Answer 6

Freezes an object. Other code cannot delete or change its properties.

Determines if an object was frozen.

Question 7

Object.is()

Answer 7

Compares if two values are the same value. Equates all NaN values (which differs from both IsLooselyEqual used by == and IsStrictlyEqual used by ===).

Question 8

Object.keys()

Object.values()

Answer 8

Returns an array containing the names of all of the given object’s own enumerable string properties.

Returns an array containing the values that correspond to all of a given object’s own enumerable string properties.

Question 9

Object.isSealed()

Object.seal()

Answer 9

Determines if an object is sealed.

Prevents other code from deleting properties of an object.

Question 10

Object.preventExtensions()

Object.isExtensible()

Answer 10

Prevents any extensions of an object.

Determines if extending of an object is allowed.

Question 11

Object.prototype.toLocaleString()

Answer 11

Calls toString().

Question 12

Object.prototype.toString()

Answer 12

Returns a string representation of the object.

Question 13

Object.prototype.valueOf()

Answer 13

Returns the primitive value of the specified object.

Question 14

JSON

Answer 14

The JSON object contains methods for parsing JavaScript Object Notation (JSON) and converting values to JSON.

Question 15

JavaScript and JSON differences

Answer 15

JSON is a syntax for serializing objects, arrays, numbers, strings, booleans, and null. It is based upon JavaScript syntax but is distinct from it: some JavaScript is not JSON.

Objects and Arrays
for JSON: property names must be double-quoted strings; trailing commas are forbidden.

Numbers
Leading zeros are prohibited. A decimal point must be followed by at least one digit. NaN and Infinity are unsupported.

Question 16

JSON.parse(text[, reviver])

Answer 16

Parse the string text as JSON, optionally transform the produced value and its properties, and return the value. Any violations of the JSON syntax, including those pertaining to the differences between JavaScript and JSON, cause a SyntaxError to be thrown.

Question 17

JSON.stringify(value[, replacer[, space]])

Answer 17

Return a JSON string corresponding to the specified value, optionally including only certain properties or replacing property values in a user-defined manner. By default, all instances of undefined are replaced with null, and other unsupported native data types are censored. The replacer option allows for specifying other behavior.

Question 18

JSON example

Answer 18

{
  "browsers": {
    "firefox": {
      "name": "Firefox",
      "pref_url": "about:config",
      "releases": {
        "1": {
          "release_date": "2004-11-09",
          "status": "retired",
          "engine": "Gecko",
          "engine_version": "1.7"
        }
      }
    }
  }
}

Question 19

JS Object Example

Answer 19

const car = {type:”Fiat”, model:”500”, color:”white”};

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
  eyeColor: "blue",
};

Question 20

Object Properties Shorthand

Answer 20

Function makeStatObj(array) {
    const Max = Math.max(…array);
    const Min = Math.min(…array);
    const sum = array.reduce((sum, value) => sum + value);

return { max, min, sum }; // object }

Question 21

How can you make computed properties for an object?

Answer 21

use a variable as a key name in an object literal property

const user = 'John'
const role = 'Admin'

const userRoles = {
    [user]: role      // John: "Admin"
}

Question 22

How can we create object methods?

Answer 22

const coolObject = {
  Name: "coolObject",
  speak() {
    console.log(`I am a cool object`);
  }
}

OR

const coolObject = {
  name: "coolObject",
  speak: function() {
    console.log(`I am a cool object`);
  }
}

Question 23

How would you use this in an object/class?

Answer 23

const coolObject = {
  first: 'cool'
  last: 'object'
  fullName() {
    return `${this.first} ${this.last};
  }
  sayHello() {
    const name = this.fullName();
    console.log(`Hi, I am ${name}!`);
  }
}

Question 24

if a function with this is reassigned in the global scope, what will ‘this’ refer to?

Answer 24

The window object

Question 25

If you use this in an arrow function (method) what will it refer to?

Answer 25

the global scope (window in the browser).
It is advised to not use arrow functions to define methods in objects/classes

instead use a method declaration to house an arrow function.
(if necessary when using global functions)

Question 26

Do arrow functions get their own ‘this’?

Answer 26

No, this will refer to the global object in an arrow function

Question 27

what will ‘this’ refer to if used in a setInterval() function?

Answer 27

The global object (window in browsers)

Question 28

how can you use a global function with this in an object/class?

Answer 28

with an arrow function, because it does not get it’s own scope.

const coolObject = {
    keyPhrase: 'I am so cool.',

repeatPhrase() {
     this.timerId = setInterval(() => { 
         console.log(this.keyPhrase);  //inherits scope from repeatPhrase()
     }, 5000)
}
stopPhrase() {
     clearInterval(this.timerId);
     console.log('cool object has stopped');
} }