JavaScript focused study deck

Question 1

What does a for…in loop return to us?

Answer 1

A for…in loop returns the keys of an object.

In the case of an Array, this would be the indices

This also works for strings

Question 2

What does a for…of loop return to us?

Answer 2

A for…of loop returns each element in the iterable.

This works for strings and arrays, but not objects

Question 3

What does the filter method return to us?

Answer 3

The filter method returns a new array with elements that match the given conditionals (the given test implemented by the given function)

Question 4

What argument must we provide to a filter method?

Answer 4

We must provide a Callback function that runs the filtering test.

This can be a function expression, an anonymous function, or an arrow function.

Question 5

Beside a callback function, what arguments can we pass to the filter method?

Answer 5

We can pass the index, and the array on which to run the callback

Question 6

What does the map method return to us?

Answer 6

The map method returns a new array populated with the results of calling a provided function on each element of the original array.

Question 7

What argument must we pass to the map method?

Answer 7

We must pass a callback function to the map method.

This callback can be a function expression, an anonymous function, or an arrow function.

Question 8

What arguments, besides a callback, with the map method take?

Answer 8

The map method will take index as an argument, as well as the array on which to run the callback.

Question 9

When shouldn’t we use the map function?

Answer 9

We shouldn’t use the map function when we don’t need the returned array.

In that case you should use a for…of loop or forEach

Question 10

Sometimes a method might return an array with undefined, NaN, or ‘ ’ at either end based on our callback.

What can we do about that?

Answer 10

We can splice the returned array to remove the index value we do not want.

Question 11

Does the splice method modify the array it is performed on?

Answer 11

Yes, it modifies the original array.

Question 12

What arguments does splice take in?

Answer 12

The splice method takes in a starting index, a delete count, and then items to be inserted.

Delete count is optional, but must be included if there are items to be inserted

Delete count can be 0

Question 13

Does the slice method alter the array it is called upon?

Answer 13

No, the slice method returns a shallow copy of the array in a new array

Question 14

What arguments does slice take?

Answer 14

Slice takes the starting index (inclusive), and the end index (exclusive)

Question 15

Does the sort method return a new array?

Answer 15

No, sort will sort the items in place, modifying the original array.

Question 16

What is the default sorting order of the sort method?

Answer 16

Ascending

Question 17

What arguments do we pass to the sort method?

Answer 17

We pass either 0 arguments for the default sort, or a callback function (expression, anonymous, or arrow).

Question 18

When comparing elements in a sort method, what determines their placement in relation to one another?

Answer 18

The return value of the compare function determines their sort order.

>0 b before a

<0 a before b

===0 keep original order

Question 19

How can we sort alphabetically with unique characters in our string or array?

Answer 19

We can use the localeCompare

a.localeCompare(b) to sort them in ascending order

Question 20

Can we use dot notation in our sort method callback function?

Answer 20

Yes, we can use dot notation to sort values in an array of object.

Question 21

Can we put conditionals in our sort method callback function?

Answer 21

Yes, we can put conditionals in our sort method callback, we need to account for return values of -1, 0, and 1

Question 22

What does the reduce method return to us?

Answer 22

A single value

Question 23

Does reduce modify the array?

Answer 23

No, not in its typical use, reduce does not modify the array.

Question 24

What arguments does reduce accept?

Answer 24

Reduce accepts an a callback function with an accumulator and a current value argument, as well as an optional initial value.

Question 25

Where does the initial value argument go in a reduce method call?

Answer 25

The initial value goes after the callback function, separated by a comma

Question 26

What arguments does the callback function passed to the reduce method accept?

Answer 26

The reduce method callback function accepts an accumulator, a current value, current index, and array arguments.

Question 27

What is a benefit of using reduce on a multi-tier array?

Answer 27

The reduce function can be used to flatten an array

Question 28

How do we flatten an array with the reduce method?

Answer 28

We have the returned value of the callback function =

previousvalue.concat(currentvalue)

Question 29

Can we use the reduce method to return a count of elements in an array?

Answer 29

Yes, we can use reduce to add names to an object that holds the elements as keys and increment a counter as the value

if(name in AllNames) {allNames[name]++} else {allNames[name]=1} return allNames