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Maciek Grzybek
Maciek Grzybek

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Repetitive tests with jest - Little Bits

Little Bits is a series of short articles explaining quick solutions to common dev problems. No unnecessary descriptions or code snippets. No bullshit.

In this short article, I want to show you how you can run repetitive test cases with Jest. This is specifically useful when testing helpers and utility methods.

Useful links:

The problem

Let's say we have a simple method for some kind of string manipulation. The function takes an original string and adds another one to it. We can also define if we want the new string to be added at the end or the beginning, as well as change it to be uppercased.

const addToString = (word, addition, placement, upperCase) => {
  let stringToReturn;

  if (placement === 'end') {
    stringToReturn = `${word}${addition}`;
  } else {
    stringToReturn = `${addition}${word}`;

  return upperCase ? stringToReturn.toUpperCase() : stringToReturn;
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To test it we could so something like that:

test('changes string node to node_js', () => {
    expect(addToString('node', 'js_')).toBe('js_node');

test('changes string node to _jsnode', () => {
    expect(addToString('node', '_js', 'end')).toBe('node_js');

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And so on with other possible scenarios.

How to solve it

Instead, we can use amazing jest method test.each:

  originalWord | addition | placement    | uppercase    | expected
  ${'node'}    | ${'js_'} | ${undefined} | ${undefined} | ${'js_node'}
  ${'node'}    | ${'js_'} | ${'end'}     | ${undefined} | ${'nodejs_'}
  ${'node'}    | ${'js_'} | ${null}      | ${true}      | ${'JS_NODE'}
  ${'node'}    | ${'js_'} | ${'end'}     | ${true}      | ${'NODEJS_'}
  'changes string $originalWord to $expected - uppercase -> $uppercase',
  ({ originalWord, addition, placement, uppercase, expected }) => {
    expect(addToString(originalWord, addition, placement, uppercase)).toBe(expected);
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As you can imagine, this works really well with bigger methods that have lots of different permutations as you can pass tens of different test scenarios while handling only one assertion. I hope that this article will help you with writing amazing tests.

Top comments (0)

In defense of the modern web

I expect I'll annoy everyone with this post: the anti-JavaScript crusaders, justly aghast at how much of the stuff we slather onto modern websites; the people arguing the web is a broken platform for interactive applications anyway and we should start over;

React users; the old guard with their artisanal JS and hand authored HTML; and Tom MacWright, someone I've admired from afar since I first became aware of his work on Mapbox many years ago. But I guess that's the price of having opinions.