Write Unit Tests for an Existing Function

This prompt generates a thorough unit test suite for a function you paste in, in the test framework you specify. It explicitly asks for coverage across the happy path, every branch, edge cases, and error conditions, which is exactly the set most hand-written tests skip under time pressure. Requiring the AAA pattern and plain-English behavioral test names produces tests that double as living documentation of what the function is supposed to do.

What you can expect back

import { describe, it, expect } from 'vitest';
import { applyDiscount } from './applyDiscount';

describe('applyDiscount', () => {
  it('applies a standard percentage discount to a positive price', () => {
    // Arrange
    const price = 100, percent = 20;
    // Act
    const result = applyDiscount(price, percent);
    // Assert
    expect(result).toBe(80);
  });

  it('returns the full price when the discount is 0 percent', () => {
    expect(applyDiscount(50, 0)).toBe(50);
  });

  it('returns 0 when a 100 percent discount is applied (boundary)', () => {
    expect(applyDiscount(50, 100)).toBe(0);
  });

  it('throws when price is negative', () => {
    expect(() => applyDiscount(-1, 10)).toThrow('price must be non-negative');
  });

  it('throws when percent is above 100 (out of range)', () => {
    expect(() => applyDiscount(100, 150)).toThrow('percent out of range');
  });
});

Illustrative example — your results will vary by tool and inputs.

Get sharper results

• 01Paste the function's real dependencies or type definitions too, so the generated tests mock collaborators correctly instead of guessing their shape.
• 02Ask the model to list which branches and edge cases it identified before writing tests, so you can confirm nothing important was missed.
• 03Tell it whether to mock external calls (network, database, time) and how, since untold assumptions here lead to flaky or non-runnable tests.
• 04Request a coverage rationale comment for tricky cases so a reviewer understands why an unusual input was chosen.
• 05Run the generated suite immediately; if a test fails, that's a signal either the test is wrong or you've found a real bug to investigate.

Adapt it for your case