Today I found myself needing to brush up on some functional programming concepts in JavaScript, namely the map(), filter(), and reduce() array methods. I found a good primer and went through the examples.

Map Operation

• used to modify the input set of array values into another output array
• does not change the values of the input array
• map function should only modify the input parameter - should not modify values outside of the map function

var namesMapMethod = names.map(function(name){
    return name.toUpperCase();
});

additionally, you can change multiple .map() functions together

var namesChained = names.map(function(name){
    return name.toUpperCase();
}).map(function(name){
    return name.replace('A', '@'); 
});

Filter Operation

• used to return a subset of array values from the input array
• does not modify the input array
• should not modify any values outside of the filter function

var filtered = names.filter(function(name){
    return (name.indexOf('a') > -1);
});

Reduce Operation

• takes an input array and returns a scalar result (unless you define the result as an array)
• can be used for string concatenation, calculating totals, etc.

var nameString = names.reduce(function(result, name){
    return result + name + "-";
}, "");

demo:

jsfiddle.net/brnvndr/94rj8nhb/