collections

Question 1

What are collections?

Answer 1

collections are made up of individual elements. In JS this includes arrays, objects, and kinda strings

Question 2

Calling theslicemethod without any arguments will return…

Answer 2

a copy of the original string

Question 3

The first argument of slice specifies…

Answer 3

the index at which to start the extraction

Question 4

The second argument of slice specifies…

Answer 4

the index where you want to end the extraction. The character at the ending index isn’t part of the returned substring.

Question 5

sliceandsubstringdiffer…

Answer 5

• When the start index is greater than the end index,substringswaps the two arguments whileslicereturns an empty string:
• When either argument is negative,substringtreats them as0, while, as we saw above,slicetreats them aslength + index:
• We recommend usingString.prototype.slice

Question 5

When given negative numbers as the indices,slicetreats them as

Answer 5

string length + index. IE, it counts backward from the last char (-1 is the last char)

Question 6

String.prototype.slicereturns…

Answer 6

a new string

Question 7

Array.prototype.slicereturns…

Answer 7

a new array

Question 7

Callingarr.slicewithout any arguments returns…

Answer 7

a shallow copy of the original array

Question 8

Referencing an out-of-bounds index with slice returns…

Answer 8

undefined

Question 9

Object keys are also called…

Answer 9

properties

Question 10

Accessing an index less than0on an array or a string returns…

Answer 10

undefined

Question 11

to check if that property exists and simply holds undefined, or does not exist, we can use…

Answer 11

let obj = { a: ‘foo’, b: ‘bar’, c: undefined};
Object.keys(obj).includes(‘c’); // => true
//OR
obj.hasOwnProperty(‘c’); // => true
obj.hasOwnProperty(‘d’); // => false

Question 11

Using a key to access a property that doesn’t exist on an object returns…

Answer 11

undefined

Question 12

We can access just the keys or just the values from an object with

Answer 12

theObject.keysandObject.valuesmethods. These methods return an array

Question 13

let capitals = { uk: ‘London’, france: ‘Paris’, germany: ‘Berlin’ };
Object.keys(capitals); // => [‘uk’, ‘france’, ‘germany’]
Object.values(capitals); // => [‘London’, ‘Paris’, ‘Berlin’]

//Whats happening here?
Object.values(capitals)[0];

Answer 13

// => ‘London’
We are making an array element reference because object.values returns an array

Question 14

JavaScript arrays are objects. The chief difference between an array and some other object is that…

Answer 14

it uses non-negative integers as its primary keys.

Question 15

What happens to an array when you add elements?

Answer 15

the array increases the value of itslengthproperty

Question 15

changing the value of thelengthproperty on an array causes…

Answer 15

the number of elements to change.

Question 15

What happens if you add add new properties to an array just as you would any other object

Answer 15

these properties are NOT elements of the array but ordinary properties and these properties will not be counted in arr.length. Those properties are are also ignored by array methods likeforEach,map, andfilter

Question 16

if you have empty elements from manually lengthening an array, iterative array methods will…

Answer 16

skip over these

Question 16

when we use anObjectmethod, such askeys on an array with non element properties…

Answer 16

we get a list of all of the property names

Question 16

when we use anObjectmethod, such askeys on an array with unset elements…

Answer 16

keys ignores the unset elements

Question 17

String.prototype.split, when called without any arguments, returns…

Answer 17

an array with the string as its only element

Question 18

When called without any arguments,Array.prototype.joinreturns…

Answer 18

a string with the elements of the array joined together into a string, separated by commas.

Question 19

Apart from theObject.keysandObject.valuesmethods, you can use_______________ to convert an object to an array.

Answer 19

Object.entries

Question 20

Can you can increment an already set array value?

Answer 20

Yes,
let numbers = [1, 2, 3, 4];
numbers[0] = numbers[0] + 1; // => 2
numbers; // => [ 2, 2, 3, 4 ]

Question 21

What happens if you increment an unset array index?

Answer 21

you end up with a NaN in that element

Question 22

string element reassignment…

Answer 22

is syntactically permitted, but doesn’t affect the string