Arrays

Question 1

first

Answer 1

_.first(array, [n]) Alias: head, take

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

_.first([5, 4, 3, 2, 1]);
=> 5

Question 2

initial

Answer 2

_.initial(array, [n])

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.

_.initial([5, 4, 3, 2, 1]);
=> [5, 4, 3, 2]

Question 3

last

Answer 3

_.last(array, [n])

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

_.last([5, 4, 3, 2, 1]);
=> 1

Question 4

rest

Answer 4

_.rest(array, [index]) Alias: tail, drop

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

_.rest([5, 4, 3, 2, 1]);
=> [4, 3, 2, 1]

Question 5

compact

Answer 5

_.compact(array)

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

_.compact([0, 1, false, 2, ‘’, 3]);
=> [1, 2, 3]

Question 6

flatten

Answer 6

_.flatten(array, [shallow])

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

_.flatten([1, [2], [3, [[4]]]]);
=> [1, 2, 3, 4];

_.flatten([1, [2], [3, [[4]]]], true);
=> [1, 2, 3, [[4]]];

Question 7

without

Answer 7

_.without(array, [*values])

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

_.without([1, 2, 1, 0, 3, 1, 4], 0, 1);
=> [2, 3, 4]

Question 8

union

Answer 8

_.union(*arrays)

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.

_.union([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2, 3, 101, 10]

Question 9

intersection

Answer 9

_.intersection(*arrays)

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.

_.intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2]

Question 10

difference

Answer 10

_.difference(array, *others)

Similar to without, but returns the values from array that are not present in the other arrays.

_.difference([1, 2, 3, 4, 5], [5, 2, 10]);
=> [1, 3, 4]

Question 11

uniq

Answer 11

_.uniq(array, [isSorted], [iterator]) Alias: unique

Produces a duplicate-free version of the array, using === to test object equality. 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 iterator function.

_.uniq([1, 2, 1, 3, 1, 4]);
=> [1, 2, 3, 4]

Question 12

zip

Answer 12

_.zip(*arrays)

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. If you’re working with a matrix of nested arrays, zip.apply can transpose the matrix in a similar fashion.

_.zip([‘moe’, ‘larry’, ‘curly’], [30, 40, 50], [true, false, false]);
=> [[“moe”, 30, true], [“larry”, 40, false], [“curly”, 50, false]]

Question 13

object

Answer 13

_.object(list, [values])

Converts arrays into objects. Pass either a single list of [key, value] pairs, or a list of keys, and a list of values.

_.object([‘moe’, ‘larry’, ‘curly’], [30, 40, 50]);
=> {moe: 30, larry: 40, curly: 50}

_.object([[‘moe’, 30], [‘larry’, 40], [‘curly’, 50]]);
=> {moe: 30, larry: 40, curly: 50}

Question 14

indexOf

Answer 14

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

Returns the index at which value can be found in the array, or -1 if value is not present in the array. Uses the native indexOf function unless it’s missing. 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 … or, pass a number as the third argument in order to look for the first matching value in the array after the given index.

_.indexOf([1, 2, 3], 2);
=> 1

Question 15

lastIndexOf

Answer 15

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

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

_.lastIndexOf([1, 2, 3, 1, 2, 3], 2);
=> 4

Question 16

sortedIndex

Answer 16

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

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 iterator is passed, it will be used to compute the sort ranking of each value, including the value you pass.

_.sortedIndex([10, 20, 30, 40, 50], 35);
=> 3

Question 17

range

Answer 17

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

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 to stop, incremented (or decremented) by step, exclusive.

_.range(10);
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
_.range(1, 11);
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
_.range(0, 30, 5);
=> [0, 5, 10, 15, 20, 25]
_.range(0, -10, -1);
=> [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
_.range(0);
=> []