Backend Tests Agent Skill

Writing Backend Tests

Recipe for adding backend (unit) tests that run without VS Code.

When to use backend tests

Use backend tests for pure logic that has no VS Code UI interaction. They are the

fastest feedback loop — no Extension Host, no display server, just Node + Mocha.

Good candidates: string manipulation, path logic, parsers, encoding, variable expansion,

data-structure helpers, environment merging.

Not suitable for: anything that calls vscode.window.*, vscode.workspace.* beyond

getConfiguration(), or depends on an active editor.

File location

test/unit-tests/backend/<name>.test.ts

Import strategy — decision tree

Module has NO transitive vscode dependency

Import directly via the @cmt/* path alias.

// encoding.test.ts — encodingUtils has no vscode imports
import { isValidUtf8 } from '@cmt/encodingUtils';

Module transitively imports vscode

Mirror the pure function logic inline in the test file. Do not import the

source module — it will fail because vscode cannot be resolved at test time

(even with the mock, deep transitive chains can break).

// expand.test.ts — expand.ts transitively depends on vscode
// Mirror of expand.substituteAll
function substituteAll(input: string, subs: Map<string, string>) {
    let finalString = input;
    let didReplacement = false;
    subs.forEach((value, key) => {
        if (value !== key) {
            const pattern = key.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
            const re = new RegExp(pattern, 'g');
            finalString = finalString.replace(re, value);
            didReplacement = true;
        }
    });
    return { result: finalString, didReplacement };
}

Add a comment like // --- Mirror of <module>.<function> --- so reviewers

can trace back to the source.

What setup-vscode-mock.ts provides

The mock is auto-loaded via Mocha's -r flag. It intercepts require('vscode') and

returns stubs for:

• workspace.getConfiguration() — returns a Proxy that yields undefined
• workspace.onDidChangeConfiguration / onDidCreateFiles / onDidDeleteFiles — no-ops
• window.createOutputChannel / showErrorMessage / showWarningMessage — no-ops
• commands.registerCommand / executeCommand — no-ops
• Position, Range, Uri — minimal implementations
• EventEmitter, Disposable, TreeItem, ThemeIcon — stubs

This lets some modules with shallow vscode dependencies work. If your module only

touches vscode.workspace.getConfiguration(), direct import may still work. Test it —

if it fails, fall back to the mirror pattern.

Test framework

| Aspect | Value |

|--------|-------|

| Runner | Mocha |

| Style | TDD — use suite() / test(), not describe() / it() |

| Assertions | Chai expect — import { expect } from 'chai' |

| Path aliases | @cmt/* → src/*, @test/* → test/* |

Skeleton — direct import pattern

import { expect } from 'chai';
import { myFunction } from '@cmt/myModule';

suite('[myFunction]', () => {
    test('does the expected thing', () => {
        const result = myFunction('input');
        expect(result).to.equal('expected');
    });

    test('handles edge case', () => {
        expect(myFunction('')).to.equal('');
    });
});

Skeleton — mirror pattern

import { expect } from 'chai';

/**
 * Tests for pure utility functions in src/someModule.ts.
 * Functions are mirrored here because someModule.ts transitively
 * depends on 'vscode'.
 */

// --- Mirror of someModule.helperFn ---
function helperFn(input: string): string {
    // Copy the implementation verbatim from the source
    return input.trim().toLowerCase();
}

suite('[helperFn]', () => {
    test('trims and lowercases', () => {
        expect(helperFn('  Hello  ')).to.equal('hello');
    });

    test('empty string', () => {
        expect(helperFn('')).to.equal('');
    });
});

Run command

yarn backendTests

Full command (for reference):

node ./node_modules/mocha/bin/_mocha \
  -u tdd \
  --timeout 999999 \
  --colors \
  -r ts-node/register \
  -r tsconfig-paths/register \
  -r test/unit-tests/backend/setup-vscode-mock.ts \
  ./test/unit-tests/backend/**/*.test.ts

Common pitfalls

| Pitfall | Fix |

|---------|-----|

| Cannot find module 'vscode' | Module has a transitive vscode dependency — use the mirror pattern |

| suite is not defined | Mocha TDD interface not loaded — ensure you run via yarn backendTests, not mocha directly |

| Import path uses relative ../../../src/ | Use @cmt/* alias instead — tsconfig-paths/register resolves it |

| describe/it used instead of suite/test | This project uses Mocha TDD style — switch to suite/test |

| Mock returns undefined for a config value | setup-vscode-mock.ts's getConfiguration() returns undefined for everything — if your code needs a real value, you may need to extend the mock or restructure |

*See also: .github/copilot-instructions.md for project-wide conventions.*