JavaScript basics · Lesson 8 of 12 · 10 minutes

JavaScript basics: Array length

length counts the array slots; the last valid index is normally length minus one.

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1
Build array length
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
length counts the array slots; the last valid index is normally length minus one.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const lessons = ["Paths", "Logs", "Services"];
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.
2
Extend the array length 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
length counts the array slots; the last valid index is normally length minus one.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const lessons = ["Paths", "Logs", "Services"];
const count = lessons.length;
```
Common mistake to avoid: A successful element lookup guarantees that the rest of an event handler is correct.
3
Verify the complete array length 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
length counts the array slots; the last valid index is normally length minus one.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
const lessons = ["Paths", "Logs", "Services"];
const count = lessons.length;
const finalLesson = lessons[count - 1];
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.

Understand the pattern

length counts the array slots; the last valid index is normally length minus one.

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 lessons = ["Paths", "Logs", "Services"];
const count = lessons.length;
const finalLesson = lessons[count - 1];

length counts the array slots; the last valid index is normally length minus one.

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 array length?length counts the array slots; the last valid index is normally length minus one.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 array length 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.

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