Jake Worth

Jake Worth

Solving Exercism's Resistor Color Duo in TypeScript

Published: October 24, 2022 • Updated: January 03, 2023 2 min read

  • typescript

Today as a code kata I completed the ‘Resistor Color Duo’ TypeScript exercise on Exercism. Here’s my work.

This exercise asks us to write a TypeScript function that takes two color names (e.g. ‘brown’ and ‘black’) and returns the electrical resistance they would produce on a resistor as a number (10 in this example).

My Goals

I approached this challenge with the following goals:

  • Solve the problem
  • Learn TypeScript better
  • Leverage implicit typing

Here’s the spec and my solution.


Here’s spec that describes the desired behavior.

import { decodedValue } from './resistor-color-duo'

describe('Resistor Colors', () => {
  it('Brown and black', () => {
    expect(decodedValue(['brown', 'black'])).toEqual(10)

  it('Blue and grey', () => {
    expect(decodedValue(['blue', 'grey'])).toEqual(68)

  it('Yellow and violet', () => {
    expect(decodedValue(['yellow', 'violet'])).toEqual(47)

  it('Orange and orange', () => {
    expect(decodedValue(['orange', 'orange'])).toEqual(33)

  it('Ignore additional colors', () => {
    expect(decodedValue(['green', 'brown', 'orange'])).toEqual(51)

And here’s my solution.

const resistances = [

export const decodedValue = ([first, last]: Array<string>) => {
  const value = [first, last].reduce(
    (prev, color) => prev + resistances.indexOf(color),

  return Number(value);


Notes on my solution.

  • I like that this solution uses implicit types. I only added types when the complier asked.
  • I like that this solution uses the coincidence that each color has a sequential resistance number that aligns with an array index.
  • I like that the function uses destructured arguments rather than slicing the array. Slicing by index can feel like a magic number, whereas naming the two colors feels explicit.
  • I like anytime I get to use reduce.
  • Looking at other solutions, I think that I could have take a more mathematical approach instead of string manipulation. That’s not how I would write this in production so I think this is an honest effort for me.

I have also solved the trio version of this problem.

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