Skip to main content

Testing a Function

Let's create a more realistic example. Say we need to format a US address. Instead of putting this function directly in a React Native component, we can make it a standalone function. This will keep our component source code simpler, and it will also make the function easier to test.

Create a formatAddress.js at the root of your project and add the following:

function formatAddress(address) {
const addressLines = [];

if (address.street2) {
addressLines.push(`${}, ${address.state} ${}`);

return addressLines.join('\n');

export default formatAddress;

Create a formatAddress.spec.js file at the root of your project and add the following:

import formatAddress from './formatAddress';

describe('formatAddress', () => {
it('returns the formatted address', () => {
const addressObject = {
street1: '123 Main Street',
street2: 'Apartment 456',
city: 'Atlanta',
state: 'GA',
zip: '30307',

const result = formatAddress(addressObject);
const expected = '123 Main Street\nApartment 456\nAtlanta, GA 30307';


Run the tests with yarn test. They should pass.

For more practice, try writing another it() that specifies what happens when there is no street2 value. Try writing another for what the function should do if there is no street1 or street2, to see the tests fail. Update the code to handle that case, rerun the tests, and watch them pass.