reduce.js

// This is not my code, I've just saved it for myself. Origin: https://github.com/leighhalliday/reduce-example

const people = [
    { id: "1", name: "Leigh", age: 35 },
    { id: "2", name: "Jenny", age: 30 },
    { id: "3", name: "Heather", age: 28 },
];

let result;

// count

result = people.reduce((acc, person) => acc + 1, 0);

// sum ages

result = people.reduce((acc, person) => acc + person.age, 0);

// array of names (map)

result = people.reduce((acc, person) => [...acc, person.name], []);

// convert to id => person lookup (dict)

result = people.reduce((acc, person) => {
    return { ...acc, [person.id]: person };
}, {});

// max age

result = people.reduce((acc, person) => {
    if (acc === null || person.age > acc) return person.age;
    return acc;
}, null);

// min age

result = people.reduce((acc, person) => {
    if (acc === null || person.age < acc) return person.age;
    return acc;
}, null);

// find by name

result = people.reduce((acc, person) => {
    if (acc !== null) return acc;
    if (person.name === "Leigh") return person;
    return null;
}, null);

// all over 18

result = people.reduce((acc, person) => {
    if (!acc) return false;
    return person.age > 18;
}, true);

// any over 18

result = people.reduce((acc, person) => {
    if (acc) return true;
    return person.age > 18;
}, false);

// count occurrences

const orders = [
    { id: "1", status: "pending" },
    { id: "2", status: "pending" },
    { id: "3", status: "cancelled" },
    { id: "4", status: "shipped" },
];

result = orders.reduce((acc, order) => {
    return { ...acc, [order.status]: (acc[order.status] || 0) + 1 };
}, {});

// flatten

const folders = [
    "index.js",
    ["flatten.js", "map.js"],
    ["any.js", ["all.js", "count.js"]],
];

function flatten(acc, item) {
    if (Array.isArray(item)) {
        return item.reduce(flatten, acc);
    }
    return [...acc, item];
}

result = folders.reduce(flatten, []);

// create reduce ourselves

function reduce(array, callback, initial) {
    let acc = initial;
    for (let i = 0; i < array.length; i++) {
        acc = callback(acc, array[i], i);
    }
    return acc;
}

result = reduce([1, 2, 3], (acc, num) => acc + num, 0);

// reduce with early abort

function reduceAbort(array, callback, initial) {
    let acc = initial;

    try {
        for (let i = 0; i < array.length; i++) {
            acc = callback(acc, array[i], i, array);
        }
    } catch (error) {
        if (Array.isArray(error) && error[0] === "abort") {
            return error[1];
        }
        throw error;
    }

    return acc;
}

person = reduceAbort(
    people,
    (_acc, person) => {
        if (person.name === "Leigh") throw ["abort", person];
        return null;
    },
    null
);