Why functions instead of methods?

Dinero.js uses standalone functions:

add(d1, d2);

Instead of chainable methods:

d1.add(d2);

The primary reason is modularity. With standalone functions, bundlers can eliminate unused code. With methods, every Dinero object would carry every operation on its prototype, including the ones you never use.

Composition

Standalone functions compose well with functional utilities:

import { pipe } from 'ramda';

pipe(
  (d) => multiply(d, 2),
  (d) => add(d, fee),
  (d) => toDecimal(d),
)(price);

Custom chaining

Nesting can get verbose and hard to understand when inlining:

// Multiplies, then adds, but looks like the opposite
add(multiply(d1, 2), d2);

If you prefer chaining, you can create your own wrapper:

function chain(d) {
  return {
    multiply: (n) => chain(multiply(d, n)),
    add: (other) => chain(add(d, other)),
  };
}

chain(d1).multiply(2).add(d2);

For most use cases, the functional style works well and keeps your bundle small.