Arrays & Objects

Question 1

How do you tell if something is an array?

Answer 1

use Array.isArray(arr) which will return a boolean value

Question 2

how do you find how many set values an array has?

Answer 2

While thelengthproperty of Array includes unset values in the count,Object.keysonly counts those values that have been set to some value:

Question 3

What happens if you change an array’slengthproperty to a new, smaller value?

Answer 3

If you change an array’slengthproperty to a new, smaller value, the array gets truncated; JavaScript removes all elements beyond the new final element.

Question 4

What is a const array?

Answer 4

variables declared withconstand initialized to an array are a little strange; while you can’t change what array the variable refers to, you can modify the array’s contents:

Question 5

how do you stop an array from being modified?

Answer 5

If you want the elements of the array to also beconst, you can use theObject.freezemethod.
(This is a shallow freeze, objects in the array will still be mutable)

Question 6

can you use === to compare arrays?

Answer 6

JavaScript treats two arrays as equal only when they are the same array: they must occupy the same spot in memory. This rule holds for JavaScript objects in general; objects must be the same object

Question 7

What is weird about includes() and indexOf() when used with nested arrays?

Answer 7

includes and indexOfinternally use===to compare elements of the array with the argument. That means we can’t useindexOf or includesto check for the existence of a nested array or an object unless we have a reference to the same object or array we’re looking for:

Question 8

What is array destructuring?

Answer 8

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

Question 9

What is the syntax for array destructuring?

Answer 9

let a, b, rest;
[a, b] = [10, 20];

console.log(a);
// Expected output: 10

Question 10

How can we access data from an object?

Answer 10

> person.name // dot notation
= ‘Jane’

> person[‘age’] // bracket notation
= 37

Question 11

If you have a variable that contains an object keys name, how must you use it?

Answer 11

you must use bracket notation
> let key = ‘name’
> person[key]

Question 12

How do you delete a property from an object?

Answer 12

> delete person.age
= true

> delete person[‘gender’]
= true

Question 13

How can we define new methods?

Answer 13

Array.prototype.push = function(newValue) {
this[this.length] = newValue;
}

Question 14

What is NOT an object or primative?

Answer 14

• variables and other identifiers such as function names
• statements such asif,return,try,while, andbreak
• keywords such asnew,function,let,const, andclass
• comments
• anything else that is neither data nor a function

Question 15

How do we create a new object that inherits from an existing object?

Answer 15

The static methodObject.createprovides a simple way to create a new object that inherits from an existing object:

let bob = { name: ‘Bob’, age: 22 };
let studentBob = Object.create(bob);
studentBob.year = ‘Senior’;

console.log(studentBob.name); // => ‘Bob’

Question 16

How can we iterate through each element in an array?

Answer 16

using forEach()

Question 17

How can we iterate through each property in an object?

Answer 17

a for/in loop

Question 18

What is one downside of using a for/in loop?

Answer 18

One feature – or downside, depending on how you look at it – offor/inis that it iterates over the properties of an object’s prototypes as well. We can get around this using the hasOwnProperty method

Question 19

What does the JS date Object do?

Answer 19

JavaScriptDateobjects represent a single moment in time in a platform-independent format.Dateobjects encapsulate an integral number that represents milliseconds since the midnight at the beginning of January 1, 1970, UTC (theepoch).

Question 20

What is a for/of loop?

Answer 20

for/ofis similar tofor/in, but it iterates over the values of any “iterable” collection. For our purposes, the only iterable collections are arrays and strings.

let arr = [ 10, 20, 30 ]
for (let value of arr) {
console.log(value);
}

Question 21

What is optional chaining?

Answer 21

function reverse_words(sentence) {
return sentence?.split(‘ ‘)
.reverse()
.join(‘ ‘);
}
In this code, the?.operator performs optional chaining. Ifsentenceis nullish, the entire chain evaluates asundefined. Otherwise, the expression is evaluated as expected (assumingsentenceis a string):