DOM and events · Lesson 2 of 10 · 7 minutes

DOM and events: Query a button

A selector should identify the intended control rather than an unrelated first button.

Open the lab Course map

Mastery 0/3Run itExplain itApply it

Current edit

PreviewNo external resources

Your objectives

Hints are always available. Wrong turns never cost XP.

1
Build query a button
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
A selector should identify the intended control rather than an unrelated first button.
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 query a button 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
A selector should identify the intended control rather than an unrelated first button.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const button = document.querySelector("#start");
const label = "Start practice";
```
Common mistake to avoid: A successful element lookup guarantees that the rest of an event handler is correct.
3
Verify the complete query a button 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
A selector should identify the intended control rather than an unrelated first button.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const button = document.querySelector("#start");
const label = "Start practice";
if (button) { button.textContent = label; }
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.

Understand the pattern

A selector should identify the intended control rather than an unrelated first button.

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 label = "Start practice";
if (button) { button.textContent = label; }

A selector should identify the intended control rather than an unrelated first button.

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 query a button?A successful element lookup guarantees that the rest of an event handler is correct.A selector should identify the intended control rather than an unrelated first button.A string containing code executes automatically when it is assigned.

Apply it elsewhere

You reuse query a button 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: Click listeners Try an incident drill

Anonymous lesson and mastery progress lasts for this session. Sign in to keep synced lesson XP between sessions; mastery stays device-local. No speed bonus, lost lives, or hint penalties.

Keep learning

MDN: JavaScript fundamentals