Underscorejs _.countby (list, iteratee, [context])

Grammar

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

Description

Sorts a list of groups, and returns a count of the number of objects in each group. Similar to GroupBy, but does not return the value of the list, but returns the number of values in that group. Just like the classified statistics in Excel

• The list is a collection of loops, such as arrays, objects, strings, arguments, and so on.
• Iteratee iterators can be either a function or a string, etc.
• Iteratee has three parameters (element, index, list)
• Iteratee need to have a return
• Context execution contexts, changed to this within iteratee

Sample code example one: according to iteratee classification statistics

var function (num) {    return num% 2 = = = 0;}); // The qualifying key is true and the non-conforming key is false.  //= Object {false:3, true:2}

Example two: The object that returns the result specifies key

var function (num) {  return num% 2 = = 0? ' Even ': ' Odd '//= = Object {odd:3, even:2}

Example three: Changing the This in the Iteratee

var function (num) {    console.log (this//5 times = = Object {no:3}    return  This  3//= Object {false:3, true:2}

Example four: iteratee three parameters & Don't forget to return

var function (element, index, list) {    Console.log (element, index, list);     // = a 0 ABC    // = b 1 ABC    // = C 2 ABC  //= Object {Undefined:3}

Iteratee can be a property of an object

var list = [' One ', ' one ', ' one ', ' three ', ' four ', ' five ', ' six ', ' seven ', ' eight ', ' nine ', ' ten ']; var grouped = _.countby (list, ' length '//= = = Object {3:4, 4:3, 5:3}

Iteratee can be the object's key

var list = [{x: ' a '}, {x: ' B '}, {x: ' A '}]; var grouped = _.countby (list, ' x '//= = = Object {a:2, b:1}

Iteratee can be an object

var list = [{x:0, y:0}, {x:0, y:1}, {x:1, y:1}]; var grouped = _.countby (list, {x:0//= = = Object {true:2, false:1}

Do not pass the iteratee to the value to classify the statistic

var parity = _.countby (' abcab '//= = Object {a:2, b:2, c:1}

Non-collection Return {}

// = = Object {} // = = Object {}Console.log (_.countby (null//= = Object {}//= =) Object {}Console.log (_.countby (true//= = Object {}Console.log (_.countby ( false // = = Object {}

Underscorejs _.countby (list, iteratee, [context])