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Digital · Guided lesson

Trace a browser interaction

About 31 minutes · 6 reading + 25 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
See this course’s project requirements

A keyboard-usable local workshop calculator with clear validation, correct totals, and an eight-case test record.

Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.

What to include

  • Two-input state trace.
  • Deliberate type conversion and blank handling.
  • Isolated tested calculation.
  • Shared submit handler for pointer and keyboard.
  • Clear feedback and eight-case test record.

Review and revise

Use these criteria to check your work and choose what to improve.

  • Totals match the stated rule.
  • Invalid input never shows a current total.
  • Keyboard and pointer paths work consistently.
  • Tests and limitations are accurately reported.
Open the full course project
Want more background?
Before you begin

The idea in everyday life

Make a small fictional workshop calculator respond reliably to user input.

The idea

An event starts a pipeline: read values, validate, calculate, display.

Follow along

Trace a browser interaction — guided practice

Follow an input through read, calculate and render functions.

Save workshop-calculator.html and open it locally in a browser; open the same file in a text editor. It is a complete original worked program. The rule is 8 units per seat, 1–6 whole seats, plus one optional 5-unit group pack. Nothing is sent or bought. Restore the supplied file before each lesson’s code variation.

  1. 1. Run the baseline

    Leave seats at 2, pack unchecked, and click Calculate.

    You should see: Materials total: 16 units.

    A visible baseline helps connect code with behavior.

  2. 2. Find the functions

    Locate readInputs, validSeats, calculate and render in the source.

    You should see: Each function has one clear responsibility.

    Separating jobs makes changes easier to reason about.

  3. 3. Trace a second input

    Set seats to 3 and check the pack; calculate.

    You should see: 3*8+5=29 units.

    Changing input should produce a predictable state transition.

  4. 4. Write the trace

    Record raw seats "3", numeric seats 3, pack true and total 29.

    You should see: The trace distinguishes text, number and Boolean.

    Many browser bugs come from mixing value types.

Now change one thing

For 4 seats with no pack, enter the total units.

Your work

What to finish

A state trace showing quantity and supply-pack choices.

Common mistake

Watch for this

Changing the displayed result by hand instead of the input/calculation.

How can I tell I understand it?

Complete the changed case, explain your result, and pass the three application checks.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1A calculator uses 7 per ticket plus one optional 3-unit fee. Two tickets with fee total?
2A result displays before the input is validated. What risk follows?
3Which function should turn accepted quantities into a total without changing the page?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
1
Finish this lesson

Pass the check. We’ll save the progress.

Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

Go to the knowledge check
Optional course overview video
Traversy MediaJavaScript DOM Crash Course - Part 1YouTube
Optional course overview

JavaScript DOM Crash Course - Part 1

From Traversy Media. Watch here or open it on YouTube .

Optional course overview. Follow the written steps below at your own pace.
Tools, templates, and lesson sources

Tools and templates

Tools open in a new tab so you can keep your place here.

Sources and further reading

Explore the references behind this lesson.

MDN: Introduction to events
Source & access notes · checked 2026-09-12

Opened and inspected on 2026-09-12: Listeners and event objects.

Access: Free public web page; no paid enrollment required.

Reuse: Public reference; link only. LifeStarter prose, exercises and files are independently authored.

MDN: DOM scripting introduction
Source & access notes · checked 2026-09-12

Opened and inspected on 2026-09-12: Element selection and text updates.

Access: Free public web page; no paid enrollment required.

Reuse: Public reference; link only. LifeStarter prose, exercises and files are independently authored.

JavaScript DOM Crash Course - Part 1
Source & access notes · checked 2026-09-12

Metadata retrieved and inspected on 2026-09-12: exact title and channel confirmed through YouTube oEmbed. Playback and full video content were not tested. Optional overview; the written lesson and supplied practice stand alone.

Access: Free public web page; no paid enrollment required.

Reuse: Link or provider embed only; do not copy video, audio or transcript.

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