L1: Collection Basics

Question 1

What is a collection

Answer 1

A structured container.
A place to store data.

Question 2

Method that returns portions of a string or array. Does not mutate

Answer 2

.slice

Question 3

What does String.prototype.slice() return?

Answer 3

returns a segment of another string.

Question 4

What happens if .slice indices are out of order like (3, 1)? (for arrays and strings)

Answer 4

Returns an empty string or empty array

Question 5

What happens with negative slice indices like (-4, -2)
Still exclusive?

Answer 5

Works from the end (last index is -1)
Yes still exclusive

Question 6

What happens if you omit the second argument in .slice

Answer 6

Captured index goes till the end

Question 6

What is the index for the last element?
What 2 arguments do you use to ensure you capture the last element?

Answer 6

-1
Not 2 arguments. not 0 or -0. Just use 1 argument

Question 6

With Array.prototype.slice(), what happens if you have no arguments? What does it return? Does it return a new array or a reference to the original?

Answer 6

A copy of the original array
new array

Question 6

What happens if you have no arguments with String.prototype.slice()?

Answer 6

Returns the original string

Question 6

What happens when you try to reference an array/string index outside what’s there?
How about negative indices? (strings vs arrays?)as with anArray[-3]

Answer 6

Undefined
Undefined
Not the value undefined, but the actual absence of the value.

With arrays, -1 is viewed as a key. undefined If it’s not there, or the value if it is.

Question 6

How do you distinguish between a non object key and an object key with the value undefined?

Answer 6

Object.prototype.hasOwnProperty()
or
Object.keys(obj).includes(‘key to test’)

Question 6

What does Object.prototype.hasOwnProperty() do?
Does the argument have to be a string?

Answer 6

Returns boolean if an object has the argument key

No, it coerces it into a string.

Question 6

What happens with Object.keys(arr) on arr = [‘foo’, ‘bar’, ‘qux’]

Answer 6

[ ‘0’, ‘1’, ‘2’,}

Question 6

What happens with arr[-1] = 374;

Answer 6

It creates a key ‘-1’

Question 6

Increment the value of an array element
What happens if you try to increment a non-index?

Answer 6

let numbers = [1, 2, 3, 4]
numbers[0] = numbers[0] + 1;
or
arr[3] += 1;

Creates a NaN in the array. Basically assigns a NaN to that index of the array.

Question 6

Reassign a character of a string

Answer 6

Can’t do this
str[0] = ‘B’;
although it won’t throw an error (for reasons we’ll learn later)

Primitives are immutable.
Have to reaassign.

Question 7

What is logged?
let arr = [‘ele1’];
arr[10] = ‘ele2’;
A
console.log(arr.length);

B
arr.forEach(ele => console.log(ele));

C
for (let element of arr) {
console.log(element);
}

D
For (let key in arr) {
console.log(arr[key]);
}
?

Answer 7

11 (the length)

for each
ele1
ele2

for element of
ele1
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
ele2

for key in
ele1
ele2

Question 8

What’s an array called when it has some empty elements?

Answer 8

a sparse array

Question 9

Do forEach or for of iterate over sparse items

Answer 9

foreach doesn’t
for of does