JavaScript basics · Lesson 2 of 12 · 7 minutes

JavaScript basics: Mutable values

let allows a value to change; choose it when reassignment is part of the task.

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1
Build mutable values
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
let allows a value to change; choose it when reassignment is part of the task.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
let completed = 0;
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.
2
Extend the mutable values 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
let allows a value to change; choose it when reassignment is part of the task.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
let completed = 0;
completed = completed + 1;
```
Common mistake to avoid: A successful element lookup guarantees that the rest of an event handler is correct.
3
Verify the complete mutable values 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
let allows a value to change; choose it when reassignment is part of the task.
The requested statements are present as code, not inside a string or comment. No arbitrary JavaScript is executed.
```
let completed = 0;
completed = completed + 1;
const remaining = 3 - completed;
```
Common mistake to avoid: A string containing code executes automatically when it is assigned.

Understand the pattern

let allows a value to change; choose it when reassignment is part of the task.

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

let completed = 0;
completed = completed + 1;
const remaining = 3 - completed;

let allows a value to change; choose it when reassignment is part of the task.

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 mutable values?let allows a value to change; choose it when reassignment is part of the task.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 mutable values 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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