Collections

Question 1

_.each(list, iteratee, [context])

Answer 1

Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed. iter: function(element, index, list) iter obj: function(value, key, list)

Question 2

_.map(list, iteratee, [context])

Answer 2

Produces a new array of values by mapping each value in list through a transformation function (iteratee). iteratee: function(value, index, list)

Question 3

_.reduce(list, iteratee, [memo], [context])

Answer 3

Boils down a list of values into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iteratee. iteratee: function(memo, value, index, list)

Question 4

_.reduceRight(list, iteratee, memo, [context])

Answer 4

The right-associative version of reduce. (Opposite direction)

Question 5

_.find(list, predicate, [context])

Answer 5

Looks through each value in the list, returning the first one that passes a truth test (predicate), or undefined if no value passes the test.

Question 6

_.filter(list, predicate, [context])

Answer 6

Looks through each value in the list, returning an array of all the values that pass a truth test (predicate).

Question 7

_.where(list, properties)

Answer 7

Looks through each value in the list, returning an array of all the values that contain all of the key-value pairs listed in properties.

Question 8

_.findWhere(list, properties)

Answer 8

Looks through the list and returns the first value that matches all of the key-value pairs listed in properties.

Question 9

_.reject(list, predicate, [context])

Answer 9

Returns the values in list without the elements that the truth test (predicate) passes. The opposite of filter.

Question 10

_.every(list, [predicate], [context])

Answer 10

Returns true if all of the values in the list pass the predicate truth test.

Question 11

_.some(list, [predicate], [context])

Answer 11

Returns true if any of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a true element is found.

Question 12

_.contains(list, value, [fromIndex])

Answer 12

Returns true if the value is present in the list. Uses indexOf internally, if list is an Array. Use fromIndex to start your search at a given index.

Question 13

_.invoke(list, methodName, *arguments)

Answer 13

Calls the method named by methodName on each value in the list. Any extra arguments passed to invoke will be forwarded on to the method invocation.

Question 14

_.pluck(list, propertyName)

Answer 14

A convenient version of what is perhaps the most common use-case for map: extracting a list of property values.

Question 15

_.max(list, [iteratee], [context])

Answer 15

Returns the maximum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked.

Question 16

_.min(list, [iteratee], [context])

Answer 16

Returns the minimum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked.

Question 17

_.sortBy(list, iteratee, [context])

Answer 17

Returns a (stably) sorted copy of list, ranked in ascending order by the results of running each value through iteratee. iteratee may also be the string name of the property to sort by (eg. length).

Question 18

_.groupBy(list, iteratee, [context])

Answer 18

Splits a collection into sets, grouped by the result of running each value through iteratee. If iteratee is a string instead of a function, groups by the property named by iteratee on each of the values.

Question 19

_.indexBy(list, iteratee, [context])

Answer 19

Given a list, and an iteratee function that returns a key for each element in the list (or a property name), returns an object with an index of each item. Just like groupBy, but for when you know your keys are unique.

Question 20

_.countBy(list, iteratee, [context])

Answer 20

Sorts a list into groups and returns a count for the number of objects in each group. Similar to groupBy, but instead of returning a list of values, returns a count for the number of values in that group.

Question 21

_.shuffle(list)

Answer 21

Returns a shuffled copy of the list, using a version of the Fisher-Yates shuffle.

Question 22

_.sample(list, [n])

Answer 22

Produce a random sample from the list. Pass a number to return n random elements from the list. Otherwise a single random item will be returned.

Question 23

_.toArray(list)

Answer 23

Creates a real Array from the list (anything that can be iterated over). Useful for transmuting the arguments object.

Question 24

_.size(list)

Answer 24

Return the number of values in the list.

Question 25

_.partition(array, predicate)

Answer 25

Split array into two arrays: one whose elements all satisfy predicate and one whose elements all do not satisfy predicate.

Question 26

Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed. iter: function(element, index, list) iter obj: function(value, key, list)

Answer 26

_.each(list, iteratee, [context])

Question 27

Produces a new array of values by mapping each value in list through a transformation function (iteratee). iteratee: function(value, index, list)

Answer 27

_.map(list, iteratee, [context])

Question 28

Boils down a list of values into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iteratee. iteratee: function(memo, value, index, list)

Answer 28

_.reduce(list, iteratee, [memo], [context])

Question 29

The right-associative version of reduce. (Opposite direction)

Answer 29

_.reduceRight(list, iteratee, memo, [context])

Question 30

Looks through each value in the list, returning the first one that passes a truth test (predicate), or undefined if no value passes the test.

Answer 30

_.find(list, predicate, [context])

Question 31

Looks through each value in the list, returning an array of all the values that pass a truth test (predicate).

Answer 31

_.filter(list, predicate, [context])

Question 32

Looks through each value in the list, returning an array of all the values that contain all of the key-value pairs listed in properties.

Answer 32

_.where(list, properties)

Question 33

Looks through the list and returns the first value that matches all of the key-value pairs listed in properties.

Answer 33

_.findWhere(list, properties)

Question 34

Returns the values in list without the elements that the truth test (predicate) passes. The opposite of filter.

Answer 34

_.reject(list, predicate, [context])

Question 35

Returns true if all of the values in the list pass the predicate truth test.

Answer 35

_.every(list, [predicate], [context])

Question 36

Returns true if any of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a true element is found.

Answer 36

_.some(list, [predicate], [context])

Question 37

Returns true if the value is present in the list. Uses indexOf internally, if list is an Array. Use fromIndex to start your search at a given index.

Answer 37

_.contains(list, value, [fromIndex])

Question 38

Calls the method named by methodName on each value in the list. Any extra arguments passed to invoke will be forwarded on to the method invocation.

Answer 38

_.invoke(list, methodName, *arguments)

Question 39

A convenient version of what is perhaps the most common use-case for map: extracting a list of property values.

Answer 39

_.pluck(list, propertyName)

Question 40

Returns the maximum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked.

Answer 40

_.max(list, [iteratee], [context])

Question 41

Returns the minimum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked.

Answer 41

_.min(list, [iteratee], [context])

Question 42

Returns a (stably) sorted copy of list, ranked in ascending order by the results of running each value through iteratee. iteratee may also be the string name of the property to sort by (eg. length).

Answer 42

_.sortBy(list, iteratee, [context])

Question 43

Splits a collection into sets, grouped by the result of running each value through iteratee. If iteratee is a string instead of a function, groups by the property named by iteratee on each of the values.

Answer 43

_.groupBy(list, iteratee, [context])

Question 44

Given a list, and an iteratee function that returns a key for each element in the list (or a property name), returns an object with an index of each item. Just like groupBy, but for when you know your keys are unique.

Answer 44

_.indexBy(list, iteratee, [context])

Question 45

Sorts a list into groups and returns a count for the number of objects in each group. Similar to groupBy, but instead of returning a list of values, returns a count for the number of values in that group.

Answer 45

_.countBy(list, iteratee, [context])

Question 46

Returns a shuffled copy of the list, using a version of the Fisher-Yates shuffle.

Answer 46

_.shuffle(list)

Question 47

Produce a random sample from the list. Pass a number to return n random elements from the list. Otherwise a single random item will be returned.

Answer 47

_.sample(list, [n])

Question 48

Creates a real Array from the list (anything that can be iterated over). Useful for transmuting the arguments object.

Answer 48

_.toArray(list)

Question 49

Return the number of values in the list.

Answer 49

_.size(list)

Question 50

Split array into two arrays: one whose elements all satisfy predicate and one whose elements all do not satisfy predicate.

Answer 50

_.partition(array, predicate)