Reducing Filter and Map with Reduce

doctors.js

var doctors = [
    { number: 1,  actor: "William Hartnell",      begin: 1963, end: 1966 },
    { number: 2,  actor: "Patrick Troughton",     begin: 1966, end: 1969 },
    { number: 3,  actor: "Jon Pertwee",           begin: 1970, end: 1974 },
    { number: 4,  actor: "Tom Baker",             begin: 1974, end: 1981 },
    { number: 5,  actor: "Peter Davison",         begin: 1982, end: 1984 },
    { number: 6,  actor: "Colin Baker",           begin: 1984, end: 1986 },
    { number: 7,  actor: "Sylvester McCoy",       begin: 1987, end: 1989 },
    { number: 8,  actor: "Paul McGann",           begin: 1996, end: 1996 },
    { number: 9,  actor: "Christopher Eccleston", begin: 2005, end: 2005 },
    { number: 10, actor: "David Tennant",         begin: 2005, end: 2010 },
    { number: 11, actor: "Matt Smith",            begin: 2010, end: 2013 },
    { number: 12, actor: "Peter Capaldi",         begin: 2013, end: 2013 }    
];

filter-map.js

doctors = doctors.filter(function(doctor) {
    return doctor.begin > 2000; // if truthy then keep item
}).map(function(doctor) {
    return { // return what new object will look like
        doctorNumber: "#" + doctor.number,
        playedBy: doctor.actor,
        yearsPlayed: doctor.end - doctor.begin + 1
    };
});

console.log(JSON.stringify(doctors, null, 4));

output.js

[
    { doctorNumber: "#9",  playedBy: "Christopher Eccleston", yearsPlayed: 1 },
    { doctorNumber: "#10", playedBy: "David Tennant",         yearsPlayed: 6 },
    { doctorNumber: "#11", playedBy: "Matt Smith",            yearsPlayed: 4 },
    { doctorNumber: "#12", playedBy: "Peter Capaldi",         yearsPlayed: 1 }
] 

reduce.js

doctors = doctors.reduce(function(memo, doctor) {
    if (doctor.begin > 2000) { // this serves as our `filter`
        memo.push({ // this serves as our `map`
            doctorNumber: "#" + doctor.number,
            playedBy: doctor.actor,
            yearsPlayed: doctor.end - doctor.begin + 1
        });
    }
    return memo;
}, []);

console.log(JSON.stringify(doctors, null, 4));

standard-reduce.js

var developers = [
    { name: "Joe", age: 23 },
    { name: "Sue", age: 28 },
    { name: "Jon", age: 32 },
    { name: "Bob", age: 24 }
], age = 0;

age = developers.reduce(function(memo, developer) {
    return memo + developer.age; // return previous total plus current age
}, 0); // initialize age with 0 that will be passed as memo

console.log("Sum of all developer ages is " + age); 
// Output:   Sum of all developer ages is 107

underscore-filter-map-chain.js

doctors = _.chain(doctors) // starts chain using the doctors array
    .filter(function(doctor) { // uses array from chain
        return doctor.begin > 2000;
    })
    .map(function(doctor) { // uses array from chain
        return {
            doctorNumber: "#" + doctor.number,
            playedBy: doctor.actor,
            yearsPlayed: doctor.end - doctor.begin + 1
        };
    })
    .value(); // gets value from chain

console.log(JSON.stringify(doctors, null, 4));

underscore-filter-map.js

doctors = _.filter(doctors, function(doctor) {
    return doctor.begin > 2000;
});

doctors = _.map(doctors, function(doctor) {
    return {
        doctorNumber: "#" + doctor.number,
        playedBy: doctor.actor,
        yearsPlayed: doctor.end - doctor.begin + 1
    };
});

console.log(JSON.stringify(doctors, null, 4));

underscore-reduce.js

doctors = _.reduce(doctors, function(memo, doctor) {
    if (doctor.begin > 2000) {
        memo.push({
            doctorNumber: "#" + doctor.number,
            playedBy: doctor.actor,
            yearsPlayed: doctor.end - doctor.begin + 1
        });
    }
    return memo;
}, []);

console.log(JSON.stringify(doctors, null, 4));

@kangalert are functions from underscore library: http://underscorejs.org/#chain