True 8.0.0



  1. To make true; shape, adjust, place, etc., exactly or accurately:

    True the wheels of a bicycle after striking a pothole.

  2. To make even, symmetrical, level, etc. (often followed by up):

    True up the sides of a door.

  3. To test your Sass code; debug, perfect, etc. (often using True):

    True your sweet plugin before you deploy.

True is a unit-testing tool for Sass code. All of the tests are written in plain Sass, and can be compiled using Dart Sass – but we also provide integration with JavaScript test runners (e.g. Mocha or Jest), for extra features and improved reporting.


In command line:

npm install --save-dev sass-true

Import in your test directory, like any other Sass file:

@use 'pkg:sass-true' as *;

If you are not using the Sass Node.js package importer, you may need to include the full path name:

// This is only an example, your path may be different
@use '../node_modules/sass-true' as *;

Or if you are using the JavaScript test runner integration:

@use 'true' as *;

One Setting

$terminal-output (boolean), defaults to true

  • true will show detailed information in the terminal for debugging failed assertions or reporting final results. This is the default, and best for compiling without a JavaScript test runner.
  • false will turn off all terminal output from Sass, though Mocha/Jest will continue to use the terminal for reporting.

If you are still using @import rather than @use, there is an import path available - which retains the legacy prefixed $true-terminal-output variable name:

// Your path may be different
@import '../node_modules/sass-true/sass/true';


True is based on common JS-testing patterns, allowing both a test-module/test syntax, and the newer describe/it for defining the structure:

@include test-module('Zip [function]') {
  @include test('Zips multiple lists into a single multi-dimensional list') {
    // Assert the expected results
    @include assert-equal(zip(a b c, 1 2 3), (a 1, b 2, c 3));

This is the same as…

@include describe('Zip [function]') {
  @include it('Zips multiple lists into a single multi-dimensional list') {
    // Assert the expected results
    @include assert-equal(zip(a b c, 1 2 3), (a 1, b 2, c 3));

Sass is able to compare values internally, meaning function-output and variable values can easily be compared and reported during Sass compilation.

CSS output tests, on the other hand, have to be compared after compilation is complete. You can do that by hand if you want (git diff is helpful for noticing changes), or you can use a test runner such as Mocha or Jest.

Output tests fit the same structure, but assertions take a slightly different form, with an outer assert mixin, and a matching pair of output and expect to contain the output-values.

// Test CSS output from mixins
@include it('Outputs a font size and line height based on keyword') {
  @include assert {
    @include output {
      @include font-size('large');

    @include expect {
      font-size: 2rem;
      line-height: 3rem;

You can optionally show a summary report in CSS and/or the command line, after the tests have completed:

@include report;

See the full documentation online for more details. See when upgrading from an older version of True.

Using Mocha, Jest, or other JS test runners

  1. Install true via npm:

    npm install --save-dev sass-true
  2. [Optional] Install Dart Sass (sass or sass-embedded), if not already installed.

    npm install --save-dev sass
  3. Write some Sass tests in test/test.scss (see above).

  4. Write a shim JS test file in test/sass.test.js:

    const path = require('node:path');
    const sassTrue = require('sass-true');
    const sassFile = path.join(__dirname, 'test.scss');
    sassTrue.runSass({ describe, it }, sassFile);
  5. Run Mocha/Jest, and see your Sass tests reported in the command line.

Note: Because of differences between Jest globals and Node globals, Dart Sass often errors when trying to compile in a Jest environment (e.g. J.getInterceptor$ax(...).map$1$1 is not a function). This can usually be fixed by installing jest-environment-node-single-context and setting testEnvironment: 'jest-environment-node-single-context' in jest.config.js. See possible related issues.

Note: Jest does not watch for changes in Sass files by default. To use jest --watch with True, add “scss” to your moduleFileExtensions setting.

You can call runSass more than once, if you have multiple Sass test files you want to run separately.

The first argument is an object with required describe and it options, and optional sass, contextLines and sourceType options.

Any JS test runner with equivalents to Mocha’s or Jest’s describe and it should be usable in the same way: just pass your test runner’s describe and it equivalents in the first argument to runSass.

The sass option is an optional string name of a Dart Sass implementation installed in the current environment (e.g. 'embedded-sass' or 'sass'), or a Dart Sass implementation instance itself. If none is provided, this defaults to 'sass'.

If True can’t parse the CSS output, it’ll give you some context lines of CSS as part of the error message. This context will likely be helpful in understanding the parse failure. By default it provides up to 10 lines of context; if you need more, you can provide a numeric contextLines option: the maximum number of context lines to provide.

The second argument is a string representing either the path to a source Sass file (passed through to Sass’ compile function), or a string of source Sass (passed through to Sass’ compileString function). By default it is expected to be a path, and sass.compile is used. To pass in a source string (and use sass.compileString), add sourceType: 'string' to your options passed in as the first argument to runSass.

The third (optional) argument to runSass accepts the same options that Sass’ compile or compileString expect (e.g. importers, loadPaths, or style), and these are passed directly through to Sass.

By default, True makes two modifications to these options. First, True’s sass path is added to the loadPaths option, so @use 'true'; works in your Sass test file. Second, if Dart Sass v1.71 or greater is installed, importers is set to an array containing the Node.js package importer, which supports pkg: imports to resolve @use and @import for external modules installed via npm or Yarn. If importers is set (even as an empty array importers: []), it will override this default importer.

Note: True requires the default Sass 'expanded' output style, and will not work if { style: 'compressed' } is used in the third argument to runSass.

Custom Importers

If you use tilde notation (e.g. @use '~accoutrement/sass/tools') or another method for importing Sass files from node_modules, you’ll need to tell runSass how to handle that. That will require writing a custom importer and passing it into the configuration for runSass:

const path = require('node:path');
const { pathToFileURL } = require('node:url');
const sassTrue = require('sass-true');

const importers = [
    findFileUrl(url) {
      if (!url.startsWith('~')) {
        return null;
      return new URL(
        pathToFileURL(path.resolve('node_modules', url.substring(1))),

const sassFile = path.join(__dirname, 'test.scss');
sassTrue.runSass({ describe, it }, sassFile, { importers });