DOM and events · Lesson 5 of 10 · 7 minutes

DOM and events: Class additions

classList.add adds a class without replacing unrelated classes already on the element.

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Your objectives

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1
Build class additions
Build this first target: the JavaScript statements shown in the example, as executable-position code rather than a string or comment. Explain how the values and control flow implement this lesson’s concept; this browser lab checks structure without executing arbitrary JavaScript.
Hint & worked example
classList.add adds a class without replacing unrelated classes already on the element.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const panel = document.querySelector("#panel");
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.
2
Extend the class additions example
Extend your file to include: the JavaScript statements shown in the example, as executable-position code rather than a string or comment. Explain how the values and control flow implement this lesson’s concept; this browser lab checks structure without executing arbitrary JavaScript.
Hint & worked example
classList.add adds a class without replacing unrelated classes already on the element.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const panel = document.querySelector("#panel");
const stateClass = "ready";
```
Common mistake to avoid: A successful element lookup guarantees that the rest of an event handler is correct.
3
Verify the complete class additions pattern
Finish the pattern with: the JavaScript statements shown in the example, as executable-position code rather than a string or comment. Explain how the values and control flow implement this lesson’s concept; this browser lab checks structure without executing arbitrary JavaScript.
Hint & worked example
classList.add adds a class without replacing unrelated classes already on the element.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const panel = document.querySelector("#panel");
const stateClass = "ready";
if (panel) { panel.classList.add(stateClass); }
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.

Understand the pattern

classList.add adds a class without replacing unrelated classes already on the element.

Misconceptions to reject

A string containing code executes automatically when it is assigned.
A successful element lookup guarantees that the rest of an event handler is correct.

This is a code-inspection lab, not a full JavaScript runtime.

Worked example and expected result

const panel = document.querySelector("#panel");
const stateClass = "ready";
if (panel) { panel.classList.add(stateClass); }

classList.add adds a class without replacing unrelated classes already on the element.

Try a variation

Adapt the final example to a different project name while retaining its structure and relationships. A different context still needs a real browser and accessibility check.

Explain it

Which statement correctly explains class additions?A successful element lookup guarantees that the rest of an event handler is correct.classList.add adds a class without replacing unrelated classes already on the element.A string containing code executes automatically when it is assigned.

Apply it elsewhere

You reuse class additions in a different project. What must still be verified?The input values, element availability, and actual behavior in a real browser.Nothing; passing a single exercise guarantees every future use.Only whether the source code has the same number of lines.

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: DOM and events: Class toggles Try an incident drill

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MDN: JavaScript fundamentals