Jake Worth

Jake Worth

Solving Exercism's Resistor Color Trio in TypeScript

Published: October 26, 2022 • Updated: January 03, 2023 2 min read

  • typescript

Today I completed the ‘Resistor Color Trio’ TypeScript exercise on Exercism. This exercise asks us to write a TypeScript function that takes two color names (e.g. ‘brown’ and ‘black’) and a power-of-ten multiplier color (e.g. ‘black’), returning the electrical resistance they would produce in string format (‘10 ohms’ in this example).

My Solution

Here’s the provided unit test:

import { decodedResistorValue } from './resistor-color-trio'

describe('Resistor Colors', () => {
  it('Orange and orange and black', () => {
    expect(decodedResistorValue(['orange', 'orange', 'black'])).toEqual(
      '33 ohms'

  it('Blue and grey and brown', () => {
    expect(decodedResistorValue(['blue', 'grey', 'brown'])).toEqual('680 ohms')

  it('Red and black and red', () => {
    expect(decodedResistorValue(['red', 'black', 'red'])).toEqual('2 kiloohms')

  it('Green and brown and orange', () => {
    expect(decodedResistorValue(['green', 'brown', 'orange'])).toEqual(
      '51 kiloohms'

  it('Yellow and violet and yellow', () => {
    expect(decodedResistorValue(['yellow', 'violet', 'yellow'])).toEqual(
      '470 kiloohms'

And my solution:

const colorMap = [

export const decodedResistorValue = ([first, second, zeros]: Array<string>): string => {
  const baseValue = `${colorMap.indexOf(first)}${colorMap.indexOf(second)}`

  let value = Number(baseValue) * 10 ** colorMap.indexOf(zeros)
  let unit = 'ohms'
  if(value >= 1000) {
    value /= 1000
    unit = `kilo${unit}`

  return `${value} ${unit}`


Here are some notes on my solution.

  • Once again, I’ve used implicit types. I’m not sure why this time the transpiler wanted to know the return type of decodedResistorValue, but didn’t on my duo solution. Probably different compiler versions.
  • I chose to interpolate the base value rather than iterate to produce it, as I did on the duo exercise. I know more about the domain now than I did then.
  • There’s a lot of string interpolation here. I think that is more readable than string than string concatenation, but it’s ugly.
  • I reassign unit and value when value is greater than 1K. Reassignment is a technique I try to avoid but sometimes I think it’s easier to read than the alternatives.
  • This solution doesn’t scale to units bigger than kiloohms.

I’m doing these exercises because I’m bullish on TypeScript! I write TypeScript more slowly than JavaScript, but once I’ve handled the types, it becomes boring, highly predictable code.

Thank you Exercism maintainers for this exercise.

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