Reduce() in depth

Answer 1

The method you pass into .reduce takes parameters previous_returned_value, current_iteration_item, current_iteration_index, context_array

This means that each time it is called, the first argument is the return value from the previous invocation

The initial value is what is passed in only the very first time.

You can read the MDN docs about it here

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return an object form which key is idx and value is Math.pow(idx,2)

• You pass in an initial empty object
• Your method adds properties to this object
• Method returns the object

e.g.

 [1, 2, 3, 4, 5].reduce((o, e) => (o[e] = Math.pow(e, 2), o), {});
 // Object {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

count how many letters are are in an array and turn it into the object

• Again pass in an initial empty object
• Method sets/adds 1 to the object based on the current letter
• Method returns the object

So again

['f', 'o', 'o', 'b', 'a', 'r'].reduce((o, e) => (o[e] = (o[e] || 0) + 1, o), {});
// Object {f: 1, o: 2, b: 1, a: 1, r: 1}

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Please note the arrow notation functions I've used exploit the comma operator to return what I want, more traditionally you might write the function as e.g. followed by evolution to my examples

function (o, e) {
    o[e] = Math.pow(e, 2);
    return o;
}
// same as
function (o, e) {
    return o[e] = Math.pow(e, 2), o;
}
// similar to
(o, e) => {
    return o[e] = Math.pow(e, 2), o;
}
// same as
(o, e) => (o[e] = Math.pow(e, 2), o);