JavaScript’s Array.reduce method is an underrated little method. Along with Array.map and Array.filter , it fits under the banner of ‘functional programming’, in that it lets you do things you would do with for loops except with functions.

Array.map maps values in one array to computed values in another

values in one array to computed values in another Array.reduce reduces values in one array into one value.

To run Array.reduce , you’ll need:

an array

a function that takes arguments of (previousValue, currentValue) and returns a new value.

and returns a new value. the initialValue

Here’s sum implemented with reduce. The input array is [1, 2, 3] , and the function adds the running sum to the current value. The starting value is 0 . The neat part is that there’s a sort of feedback loop going: what you return from the function is given back to it for the next element.

[ 1 , 2 , 3 ]. reduce ( function ( sum , value ) { return sum + value ; }, 0 );

Reduce feels a lot to me like making juice: you have

the ingredients

a function that combines the current mixture with the next ingredient

a glass

[ ' banana ' , ' cherries ' , ' orange ' ]. reduce ( function ( juice , fruit ) { return juice + fruit ; }, empty );

Excruciating Technical Detail

Array.reduce is underrated because it can easily implement a lot of other functions with just itself: the ‘single value’ it reduces into can be anything: a number, and array, an object, and so on…

// Array.find function find ( array , tester ) { return array . reduce ( function ( memo , value ) { return memo || tester ( value ); }, false ); } find ([ 1 , 2 , 3 ], function ( value ) { return value > 2 ; }); // Array.map function map ( array , fn ) { return array . reduce ( function ( memo , value ) { return memo . concat ([ fn ( value )]); }, []); } map ([ 1 , 2 , 3 ], function ( value ) { return value + 1 ; }); // Array.filter function filter ( array , fn ) { return array . reduce ( function ( memo , value ) { return fn ( value ) ? memo . concat ([ value ]) : memo ; }, []); } filter ([ 1 , 2 , 3 ], function ( value ) { return value > 1 ; }); // .length function length ( array ) { return array . reduce ( function ( length , value ) { return ++ length ; }, 0 ); } length ([ 1 , 2 , 3 ]); // zip function zip ( array ) { return array . reduce ( function ( obj , value ) { obj [ value [ 0 ]] = value [ 1 ]; return obj ; }, {}); } zip ([[ ' foo ' , ' bar ' ], [ ' apples ' , ' grapes ' ]]);

(live examples of these working)

See Also