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Types of Testing (Functional, Non-Functional)

In short

An app can do everything it promises and still be painful: slow, confusing or easy to hack. Testing covers both what it does and how well it does it.

Functional testing verifies the system does what it's supposed to do; non-functional testing verifies how well it does it — performance, usability, security — both are essential for a genuinely quality product.

The purple blob sits on a shiny new scooter, squeezing the brake and pressing the horn.

Think of it like this

Test-riding a new scooter

First you check it starts, brakes, the horn works and the indicators blink. That's functional. Then you ask: what's the mileage, is it comfortable with two people, does it struggle on a slope? That's non-functional.

Watch it happen

Two questions every tester asks

Functional

Does login accept correct details?
Does checkout apply the coupon?
Checked against written requirements

Non-functional

Does the page load quickly?
Can a screen reader use it?
Is the password stored safely?

You need both. A feature that works but takes 20 seconds or leaks data still fails users.

The big ideas

What you need to know

Functional: what it does

Checks features against requirements. Includes unit, integration, system, smoke and user acceptance testing.

Non-functional: how well

Covers performance, security, usability, accessibility, compatibility and reliability.

Non-functional needs numbers

'Fast' isn't testable. 'Search results in under 2 seconds for 500 users' is.

Test levels stack up

Unit tests check small pieces, integration tests check pieces together, system tests check the whole app.

Save yourself the pain

Common mistakes

Oops

Only testing whether buttons work and calling it done.

Do this instead

Also ask about speed, security, accessibility and different devices. Users notice those just as much.

Oops

Writing non-functional requirements like 'the app should be fast'.

Do this instead

Push for measurable targets with numbers so you can actually pass or fail them.

Your mission

Sort the checks

~20 min

Quick check

Which of these is a non-functional test?

Go deeper

1Classify 5 sample test scenarios as functional or non-functional.
2Learn why non-functional testing is easy to under-invest in under deadline pressure.
3Preview later phases (10, 12) covering non-functional testing types in depth.