DOM and events · Lesson 6 of 10 · 7 minutes

DOM and events: Class toggles

classList.toggle alternates membership unless given an explicit force boolean.

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Your objectives

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1
Build class toggles
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.toggle alternates membership unless given an explicit force boolean.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const button = document.querySelector("#start");
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.
2
Extend the class toggles 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.toggle alternates membership unless given an explicit force boolean.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const button = document.querySelector("#start");
const panel = document.querySelector("#panel");
```
Common mistake to avoid: A successful element lookup guarantees that the rest of an event handler is correct.
3
Verify the complete class toggles 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.toggle alternates membership unless given an explicit force boolean.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const button = document.querySelector("#start");
const panel = document.querySelector("#panel");
if (button && panel) { button.addEventListener("click", () => { panel.classList.toggle("expanded"); }); }
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.

Understand the pattern

classList.toggle alternates membership unless given an explicit force boolean.

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 button = document.querySelector("#start");
const panel = document.querySelector("#panel");
if (button && panel) { button.addEventListener("click", () => { panel.classList.toggle("expanded"); }); }

classList.toggle alternates membership unless given an explicit force boolean.

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 toggles?classList.toggle alternates membership unless given an explicit force boolean.A string containing code executes automatically when it is assigned.A successful element lookup guarantees that the rest of an event handler is correct.

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: DOM and events: Data attributes Try an incident drill

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