0x20Lesson 3 of 16

Types, operators and tricky conversions

Predict what an expression produces before using it in a real program.

18 min 3-question quiz 1 code exercise

By the end of this lesson you can

Distinguish strings, numbers, booleans, null and undefined
Use arithmetic, comparison and logical operators
Spot accidental string concatenation and NaN

A value has a type. Numbers do arithmetic; strings hold text; booleans answer yes/no questions. undefined often means a value was never supplied, while null is an intentional empty value. Use typeof to inspect a value. JavaScript converts types in some expressions, so predict the type before the result.

types.js

1console.log(typeof 42);
2console.log(typeof "42");
3console.log(Number("42") + 1);
4console.log("42" + 1);

Output

number
string
43
421

Try it

Up to 15 XP

Sort the values

Before clicking, predict which values are numbers, strings, booleans or empty markers.

0 of 5 sortedScore 0/0

“0”
“"0"”
“false”
“null”
“undefined”

Use === for strict equality. && and || short-circuit: they may skip the right side. ?? uses a fallback only for null or undefined, so a valid zero stays zero. Number("oops") produces NaN; check it with Number.isNaN(value).

defaults.js

1const seats = 0;
2console.log(seats || 10);
3console.log(seats ?? 10);
4console.log(Number.isNaN(Number("oops")));

Output

10
0
true

Key takeaways

Inspect types with typeof and convert input deliberately.
=== compares without coercion.
?? preserves zero and false; || treats them as fallback cases.

Lesson quiz

3 questions · pass with 3 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write JavaScript

Write the program, run it against the test cases, and use the feedback to build your JavaScript skills.

Exercise 1

Calculate a reliable cart total

+25 XP

Input is JSON with price, quantity and optional discount. Convert the numeric text, treat a missing discount as zero, and print the total with two decimal places. If price or quantity is invalid, print Invalid cart.

Normal cart
No discount
Bad price

main.js

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JavaScript runs in a sandboxed browser frame with a 1.5 second time limit. Input is available as the string variable input.

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