ES5 + Lodash (Coderpad)

Question 1

Max value of a collection or object

Answer 1

_.max({ 5: ‘a’, 3: ‘b’ }, (value, key) => key)

Question 2

Repeat a string X times

Answer 2

_.repeat('*', 3);
// → '***'

_.repeat(‘abc’, 2);
// → ‘abcabc’
~~~

_.repeat(‘abc’, 0);
// → ‘’
~~~

Question 3

Are any elements of this object or truthy?

Answer 3

_.some(collection)

The default value of the predicate parameter is _.identity.

Question 4

Create the range [-1, 0, 1]

Answer 4

_.range(-1, 2)

Optional third param: step

Question 5

Remove an array element from an array.

Answer 5

_.reject(collection, arr => _.isEqual(arr, toRemove))

• _.without() uses ES SameValueZero, which compares if objects are the exact same thing.*
• _.remove mutates!*

Question 6

Check if n is between 0 and 4.

Answer 6

_.inRange(n, 0, 5)

Ranges seem to be exclusive.

Question 7

Add an array to an array without mutation

Answer 7

[1, [0, 1]].concat([[0, 0]])

=> [1, [0, 1], [0, 0]]

Question 8

Generate a new string with a char at index i replaced

Answer 8

str.slice(0, i) + newChar + str.slice(i + 1, str.length)

Question 9

Add a list of elements to an array, return the array

Answer 9

List([1, 2]).push(1, 2)

Question 10

Class with a class method

Answer 10

class Dog {
 static bark() {}
}

Question 11

Class with a class variable

Answer 11

class Dog {}
Dog.type = 'awesome'

Question 12

Range from 0 to n, inclusive

Answer 12

Range(0, n + 1)

Question 13

Range from 0 to n by 2, inclusive

Answer 13

Range(0, n + 1, 2)

Question 14

Range from n to 0, inclusive

Answer 14

Range(n, -1) // end is always exclusive

Question 15

Repeat value ‘a’ n times

Answer 15

Repeat(‘a’, 5)

Question 16

Get from a hash, or default to 0

Answer 16

Map().get(‘a’, 0)

Question 17

Head/first, rest, initial, last?

Answer 17

Yup, all those are methods.

Question 18

Falsey values?

Answer 18

false 0 “” null undefined NaN

Question 19

Cast to int

Answer 19

Number.parseInt(‘13a’) // Forgiving: NaN only when first char isn’t number

=> 13

Question 20

Inspect class/instance methods, exclude inherited

Answer 20

Object.getOwnPropertyNames(Number)

Object.getOwnPropertyNames(Number.prototype)

Question 21

Inspect instance methods, include inherited

Answer 21

// Hard. Use Object.getOwnPropertyNames

Question 22

Switch statement

Answer 22

switch (obj) {
 case value: statement; break;
 case value: statement; break;
 default: statement; break;
 // Under the hood: obj === value
}

Question 23

Read file

Answer 23

fs.readFile(file, ‘utf8’, (err, file) => {})

Question 24

Read lines of file

Answer 24

fs.readFile(file, ‘utf8’, (err, file) => { file.split(“\n”) })

Question 25

Trim whitespace from both sides of a string

Answer 25

” a “.trim()

Question 26

Trim X from end of string

Answer 26

‘xxabcxx’.replace(/x+$/, ‘’)
=> ‘xxabc’

Question 27

Detect if beginning/end of string matches

Answer 27

‘abc’.endsWith(‘c’) // startsWith

Question 28

Create a RegExp with interpolation

Answer 28

new RegExp(str, “gim”) // global, ignore case, multiline

Question 29

Detect if string matches a RegExp

Answer 29

/\w\s(\w+)/.test(“Jason Benn”) // RegExp method

Question 30

Grab nth match group from string

Answer 30

/\w+\s(\w+)/.exec(‘Jason Benn’)[1]

Question 31

Find index of regex match

Answer 31

“Jason Benn”.search(/\w\s(\w+)/) // Str method

Question 32

Sort a map by a criteria

Answer 32

Map().sortBy((v, k, iter) => {})

Question 33

Filter a map by a criteria

Answer 33

map.filter((v, k, iter) => {})

Question 34

All elements of an array are truthy

Answer 34

[].every()

Question 35

Any element of an array is truthy

Answer 35

[].some()

Question 36

Initialize a date to a date object

Answer 36

new Date(‘…’)

Question 37

Display a date in a particular format

Answer 37

d.getHours(), d.getDate(), d.getMonth()+1, d.getFullYear()