Arrays

Question 1

_.first(array, [n])

Answer 1

Returns the first element of an array. Passing n will return the first n elements of the array.

Question 2

_.initial(array, [n])

Answer 2

Returns everything but the last entry of the array. Especially useful on the arguments object. Pass n to exclude the last n elements from the result.

Question 3

_.last(array, [n])

Answer 3

Returns the last element of an array. Passing n will return the last n elements of the array.

Question 4

_.rest(array, [index])

Answer 4

Returns the rest of the elements in an array. Pass an index to return the values of the array from that index onward.

Question 5

_.compact(array)

Answer 5

Returns a copy of the array with all falsy values removed. In JavaScript, false, null, 0, “”, undefined and NaN are all falsy.

Question 6

_.flatten(array, [shallow])

Answer 6

Flattens a nested array (the nesting can be to any depth). If you pass shallow, the array will only be flattened a single level.

Question 7

_.without(array, *values)

Answer 7

Returns a copy of the array with all instances of the values removed.

Question 8

_.union(*arrays)

Answer 8

Computes the union of the passed-in arrays: the list of unique items, in order, that are present in one or more of the arrays.

Question 9

_.intersection(*arrays)

Answer 9

Computes the list of values that are the intersection of all the arrays. Each value in the result is present in each of the arrays.

Question 10

_.difference(array, *others)

Answer 10

Returns the values from array that are not present in the other arrays.

Question 11

_.uniq(array, [isSorted], [iteratee])

Answer 11

Produces a duplicate-free version of the array, using === to test object equality. In particular only the first occurence of each value is kept. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass an iteratee function.

Question 12

_.zip(*arrays)

Answer 12

Merges together the values of each of the arrays with the values at the corresponding position. Useful when you have separate data sources that are coordinated through matching array indexes.

Question 13

_.unzip(*arrays)

Answer 13

Given a number of arrays, returns a series of new arrays, the first of which contains all of the first elements in the input arrays, the second of which contains all of the second elements, and so on.

Question 14

_.object(list, [values])

Answer 14

Converts arrays into objects. Pass either a single list of [key, value] pairs, or a list of keys, and a list of values. If duplicate keys exist, the last value wins.

Question 15

_.indexOf(array, value, [isSorted])

Answer 15

Returns the index at which value can be found in the array, or -1 if value is not present in the array. If you’re working with a large array, and you know that the array is already sorted, pass true for isSorted to use a faster binary search

Question 16

_.lastIndexOf(array, value, [fromIndex])

Answer 16

Returns the index of the last occurrence of value in the array, or -1 if value is not present. Pass fromIndex to start your search at a given index.

Question 17

_.sortedIndex(list, value, [iteratee], [context])

Answer 17

Uses a binary search to determine the index at which the value should be inserted into the list in order to maintain the list’s sorted order. If an iteratee function is provided, it will be used to compute the sort ranking of each value, including the value you pass.

Question 18

_.findIndex(array, predicate, [context])

Answer 18

Similar to _.indexOf, returns the first index where the predicate truth test passes; otherwise returns -1.

Question 19

_.findLastIndex(array, predicate, [context])

Answer 19

Like _.findIndex but iterates the array in reverse, returning the index closest to the end where the predicate truth test passes.

Question 20

_.range([start], stop, [step])

Answer 20

A function to create flexibly-numbered lists of integers, handy for each and map loops. start, if omitted, defaults to 0; step defaults to 1. Returns a list of integers from start (inclusive) to stop (exclusive), incremented (or decremented) by step, exclusive. Note that ranges that stop before they start are considered to be zero-length instead of negative — if you’d like a negative range, use a negative step.

Question 21

Returns the first element of an array. Passing n will return the first n elements of the array.

Answer 21

_.first(array, [n])

Question 22

Returns everything but the last entry of the array. Especially useful on the arguments object. Pass n to exclude the last n elements from the result.

Answer 22

_.initial(array, [n])

Question 23

Returns the last element of an array. Passing n will return the last n elements of the array.

Answer 23

_.last(array, [n])

Question 24

Returns the rest of the elements in an array. Pass an index to return the values of the array from that index onward.

Answer 24

_.rest(array, [index])

Question 25

Returns a copy of the array with all falsy values removed. In JavaScript, false, null, 0, “”, undefined and NaN are all falsy.

Answer 25

_.compact(array)

Question 26

Flattens a nested array (the nesting can be to any depth). If you pass shallow, the array will only be flattened a single level.

Answer 26

_.flatten(array, [shallow])

Question 27

Returns a copy of the array with all instances of the values removed.

Answer 27

_.without(array, *values)

Question 28

Computes the union of the passed-in arrays: the list of unique items, in order, that are present in one or more of the arrays.

Answer 28

_.union(*arrays)

Question 29

Computes the list of values that are the intersection of all the arrays. Each value in the result is present in each of the arrays.

Answer 29

_.intersection(*arrays)

Question 30

Returns the values from array that are not present in the other arrays.

Answer 30

_.difference(array, *others)

Question 31

Produces a duplicate-free version of the array, using === to test object equality. In particular only the first occurence of each value is kept. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass an iteratee function.

Answer 31

_.uniq(array, [isSorted], [iteratee])

Question 32

Merges together the values of each of the arrays with the values at the corresponding position. Useful when you have separate data sources that are coordinated through matching array indexes.

Answer 32

_.zip(*arrays)

Question 33

Given a number of arrays, returns a series of new arrays, the first of which contains all of the first elements in the input arrays, the second of which contains all of the second elements, and so on.

Answer 33

_.unzip(*arrays)

Question 34

Converts arrays into objects. Pass either a single list of [key, value] pairs, or a list of keys, and a list of values. If duplicate keys exist, the last value wins.

Answer 34

_.object(list, [values])

Question 35

Returns the index at which value can be found in the array, or -1 if value is not present in the array. If you’re working with a large array, and you know that the array is already sorted, pass true for isSorted to use a faster binary search

Answer 35

_.indexOf(array, value, [isSorted])

Question 36

Returns the index of the last occurrence of value in the array, or -1 if value is not present. Pass fromIndex to start your search at a given index.

Answer 36

_.lastIndexOf(array, value, [fromIndex])

Question 37

Uses a binary search to determine the index at which the value should be inserted into the list in order to maintain the list’s sorted order. If an iteratee function is provided, it will be used to compute the sort ranking of each value, including the value you pass.

Answer 37

_.sortedIndex(list, value, [iteratee], [context])

Question 38

Similar to _.indexOf, returns the first index where the predicate truth test passes; otherwise returns -1.

Answer 38

_.findIndex(array, predicate, [context])

Question 39

Like _.findIndex but iterates the array in reverse, returning the index closest to the end where the predicate truth test passes.

Answer 39

_.findLastIndex(array, predicate, [context])

Question 40

A function to create flexibly-numbered lists of integers, handy for each and map loops. start, if omitted, defaults to 0; step defaults to 1. Returns a list of integers from start (inclusive) to stop (exclusive), incremented (or decremented) by step, exclusive. Note that ranges that stop before they start are considered to be zero-length instead of negative — if you’d like a negative range, use a negative step.

Answer 40

_.range([start], stop, [step])