DOM and events · Lesson 1 of 10 · 7 minutes

DOM and events: Query one element

querySelector returns the first matching element or null. Guard a lookup before updating an optional element.

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Your objectives

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1
Build query one element
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
querySelector returns the first matching element or null. Guard a lookup before updating an optional element.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const heading = document.querySelector("h1");
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.
2
Extend the query one element 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
querySelector returns the first matching element or null. Guard a lookup before updating an optional element.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const heading = document.querySelector("h1");
const message = "Workshop ready";
```
Common mistake to avoid: A successful element lookup guarantees that the rest of an event handler is correct.
3
Verify the complete query one element 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
querySelector returns the first matching element or null. Guard a lookup before updating an optional element.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const heading = document.querySelector("h1");
const message = "Workshop ready";
if (heading) { heading.textContent = message; }
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.

Understand the pattern

querySelector returns the first matching element or null. Guard a lookup before updating an optional 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 heading = document.querySelector("h1");
const message = "Workshop ready";
if (heading) { heading.textContent = message; }

querySelector returns the first matching element or null. Guard a lookup before updating an optional 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 query one element?A string containing code executes automatically when it is assigned.A successful element lookup guarantees that the rest of an event handler is correct.querySelector returns the first matching element or null. Guard a lookup before updating an optional element.

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

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