# Testing coloured output

> Assert what your CLI prints at each colour level by flying a literal runtime — no environment patching, no fake terminal.

Source: https://roundel.interlace.tools/docs/recipes/testing-colour

The policy reads only the runtime it is given, so a test flies a literal and asserts the exact
bytes, at any level, on any machine, in any CI:

```js title="colours.mjs"
import assert from 'node:assert/strict';

import { fly } from 'roundel/theme';
import { error } from 'roundel/tokens';

const at = (env, tty = false) => fly({ ground: '#ffffff', error: '#b3261e' }, { env, isTTY: { stdout: tty } });

at({ FORCE_COLOR: '3' });
assert.equal(error('x'), '\u001B[38;2;179;38;30mx\u001B[39m');

at({ FORCE_COLOR: '2' });
assert.equal(error('x'), '\u001B[38;5;124mx\u001B[39m');

at({ TERM: 'xterm-256color' });
assert.equal(error('x'), 'x');

at({ TERM: 'xterm-256color' }, true);
assert.equal(error('x'), '\u001B[38;5;124mx\u001B[39m');

at({ NO_COLOR: '1', COLORTERM: 'truecolor' }, true);
assert.equal(error('x'), 'x');

console.log('ok');
```

```text title="node colours.mjs"
ok
```

Each `fly()` replaces the last, so the order of the cases does not matter. The same literal
runtime answers `outputMode()` too, which is how a test checks that a component redraws on a
terminal and writes plain lines everywhere else.