What is Software Testing

Core Definition

Why Testing Matters in Production

• Defect Prevention: Finding bugs in development costs 10x less than in production
• User Trust: Untested software erodes user confidence and brand reputation
• Business Risk: A single critical bug can cost millions (NASA Mars Orbiter: $327M lost due to a unit error)
• Security: Untested authentication logic creates exploitable vulnerabilities
• Compliance: Industries like healthcare (FDA) and finance (PCI-DSS) mandate testing
• Performance: Testing ensures systems handle real-world load without crashing

Testing in Real Software Development

// Real-world example: An e-commerce checkout function
// WITHOUT testing:
function applyDiscount(price, discountPercent) {
    return price - (price * discountPercent / 100);
}
// Bug: what if discountPercent = 150? Customer gets money BACK.
// This exact bug caused a $5M loss for a retail company in 2022.

// WITH testing — the bug is caught before production:
function applyDiscount(price, discountPercent) {
    if (discountPercent < 0 || discountPercent > 100) {
        throw new Error("Discount must be between 0 and 100");
    }
    return price - (price * discountPercent / 100);
}

// Test that catches the bug:
test("applyDiscount should reject discount over 100%", () => {
    expect(() => applyDiscount(100, 150)).toThrow("Discount must be between 0 and 100");
});

test("applyDiscount calculates correctly", () => {
    expect(applyDiscount(100, 20)).toBe(80);   // 20% off $100 = $80
    expect(applyDiscount(50, 0)).toBe(50);      // 0% off = no change
    expect(applyDiscount(200, 100)).toBe(0);    // 100% off = free
});

Testing Logic

• The tester's job is to BREAK the system — think like an attacker, not a builder
• Every function has a contract: given these inputs → expect this output — testing verifies the contract
• A test that never fails is worthless — it must be capable of detecting real defects
• Good testing asks: what happens at boundaries, with invalid data, under load, and in concurrent scenarios?

Common Mistakes

• Testing only the happy path — most bugs live in edge cases and error paths
• Assuming the spec is correct — requirements themselves can be wrong; testers must question them
• Not testing after fixes — every bug fix can introduce a regression; always retest
• Treating testing as the last phase — shift-left means testing starts from requirements

Tip

Practice Task

Quick Quiz

Common Mistake