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Jest's toEqual matcher takes whitespace into account when checking for equality. When formatting the expected value in tests it is impossible to do so in a way that matches a string containing newlines, tabs etc.

Does Jest offer a way to disregard whitespace when matching?

Jest's toEqual matcher takes whitespace into account when checking for equality. When formatting the expected value in tests it is impossible to do so in a way that matches a string containing newlines, tabs etc.

Does Jest offer a way to disregard whitespace when matching?

• javascript
• string
• jestjs
• matcher
• html-escape-characters

Share Improve this question Follow edited Dec 13, 2023 at 3:30 Rimian 38.4k1717 gold badges124124 silver badges119119 bronze badges asked Jan 25, 2018 at 19:26 UndistractionUndistraction 43.6k6262 gold badges205205 silver badges335335 bronze badges 1

• What does flatErrorMessage look like? And what do you test? – Kosh Commented Jan 25, 2018 at 19:40

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3 Answers 3

Sorted by: Reset to default 15

As @Timo says, the only way of doing this appears to be with a custom matcher. Here is one that compresses all whitespace down to a single space for readability based on Jest's toEqual matcher. It will deal with tabs, newlines etc. It will give you pretty output like the included Jest matchers:

//matchStringIgnoringWhiteSpace.js

import { replace, map, equals } from 'ramda';
import { matcherHint, printReceived, printExpected } from 'jest-matcher-utils';
import diff from 'jest-diff';

const replaceWhitespace = replace(/\s+/g, ` `);
const compressWhitespace = map(replaceWhitespace);

const name = `toEqualWithCompressedWhitespace`;

export default function (received, expected) {
  const [
    receivedWithCompresssedWhitespace,
    expectedWithCompresssedWhitespace,
  ] = compressWhitespace([received, expected]);
  const pass = equals(
    receivedWithCompresssedWhitespace,
    expectedWithCompresssedWhitespace
  );
  const message = pass
    ? () =>
        `${matcherHint(`.not.${name}`)}\n\n` +
        `Uncompressed expected value:\n` +
        `  ${printExpected(expected)}\n` +
        `Expected value with compressed whitespace to not equal:\n` +
        `  ${printExpected(expectedWithCompresssedWhitespace)}\n` +
        `Uncompressed received value:\n` +
        `  ${printReceived(received)}\n` +
        `Received value with compressed whitespace:\n` +
        `  ${printReceived(receivedWithCompresssedWhitespace)}`
    : () => {
        const diffString = diff(
          expectedWithCompresssedWhitespace,
          receivedWithCompresssedWhitespace,
          {
            expand: this.expand,
          }
        );
        return (
          `${matcherHint(`.${name}`)}\n\n` +
          `Uncompressed expected value:\n` +
          `  ${printExpected(expected)}\n` +
          `Expected value with compressed whitespace to equal:\n` +
          `  ${printExpected(expectedWithCompresssedWhitespace)}\n` +
          `Uncompressed received value:\n` +
          `  ${printReceived(received)}\n` +
          `Received value with compressed whitespace:\n` +
          `  ${printReceived(receivedWithCompresssedWhitespace)}${
            diffString ? `\n\nDifference:\n\n${diffString}` : ``
          }`
        );
      };
  return {
    actual: received,
    expected,
    message,
    name,
    pass,
  };
};

To register the custom matcher you need to add it to your setupTests files. First register setupTests in your jest.config.js using the setupFilesAfterEnv field:

 setupFilesAfterEnv: `<rootDir>/path/to/setupTests.js`,

And then register the custom matcher on the expect object.

//setupTests.js

import toMatchStringIgnoringWhitespace from "<rootDir>/path/to/matchStringIgnoringWhiteSpace";

expect.extend({
    toMatchStringIgnoringWhitespace: toMatchStringIgnoringWhitespace
}); 

If you are using TypeScript you will also want to add the typings to the expect object following the instructions here.

As far as I know, there is no way to achieve this with Jest out of the box.

However, you can write your own reusable matcher using expect.extend. Remove all whitespace from both strings, e.g. via str.replace(/\s/g, ''), and compare the strings.

While this is not a direct answer, you can also do:

mockedFunction.mock.calls[0] // To get array of arguments

// or 

mockedFunction.mock.calls[0][0] // to get first argument and so on

And then compare with equality.

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