Numbers & Math

Prices, scores, percentages, coordinates, timers — numbers run through every app. JavaScript has just one number type for integers and decimals alike, which is convenient until you hit the famous 0.1 + 0.2 surprise.

Learn Numbers & Math in our free JavaScript course — a beginner-friendly interactive lesson with runnable examples, a practice exercise and a quick recall.

Part of the free JavaScript course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

This lesson covers arithmetic, rounding, the Math object, conversions, and how to handle precision like a professional.

What You'll Learn in This Lesson

📐 Real-World Analogy: Think of a kitchen scale that only shows two decimal places:

• • The scale stores far more digits internally than it shows you
• • Tiny rounding at the last digit is invisible until you add many readings
• • For a recipe (display) you round; for selling by exact weight (money) you use whole grams
• • That's exactly why we round for display and use integer cents for currency

🧠 Arithmetic & the Modulo Operator

The basic operators are + - * / , plus % (remainder) and ** (exponent). The remainder operator is the secret behind "is this even?" and "wrap around a clock":

⚠️ The Floating-Point Gotcha

Every JavaScript number is a 64-bit float, so some decimals can't be stored exactly. This is not a bug in your code — it happens in nearly every programming language:

🔘 Rounding the Right Way

Different jobs need different rounding. Pick deliberately — the wrong one silently corrupts totals:

Note the negative trap: Math.floor(-4.7) is -5 because floor always moves toward negative infinity, while Math.trunc just deletes the fractional part.

🧮 The Math Object & Random Numbers

Math is a built-in toolbox of static helpers. You don't create it — you call methods on it directly. Random integers are the most-asked-for recipe:

🔁 Converting Strings to Numbers

User input always arrives as text. Convert it carefully — the method you choose changes the result:

🚫 NaN, Infinity & Safe Checks

Two special numeric values appear when math goes sideways. Knowing how to detect them prevents hard-to-trace bugs:

🧩 Reorder Challenge

These lines should compute a price with 20% tax and print it as currency. They are scrambled — reorder them so the log is $24.00 .

Why: the base price (C) must exist before tax is computed from it (A). The total (D) adds the two together, and only then can it be formatted (B). Working in integer cents avoids the floating-point error you'd hit doing 20 * 1.2 directly, and Math.round keeps the tax a whole number of cents.

🧠 Quick Recall

55 — Number("5") is the number 5, but 5 + "5" triggers string concatenation (the number is coerced to text), producing "55" . If you wanted 10, both operands must be numbers.

-3 — Math.floor always rounds toward negative infinity, so -2.1 becomes -3 , not -2 . Use Math.trunc(-2.1) if you want -2 .

false — 0 / 0 is NaN , and NaN is the only value in JavaScript that is not equal to itself. That's exactly why you must use Number.isNaN(x) to detect it.

⚠️ Common Mistakes to Avoid

• 1️⃣ Comparing decimals with ===. 0.1 + 0.2 === 0.3 is false ; round or use an epsilon.
• 2️⃣ Comparing to NaN. x === NaN never works — use Number.isNaN(x) .
• 3️⃣ Forgetting Number("") is 0. Empty input quietly becomes zero, hiding validation bugs.
• 4️⃣ Using floats for money. Store cents as integers and divide only when displaying.

Frequently Asked Questions

Lesson Complete — Numbers & Math!

• ✅ One number type covers integers and decimals; % gives the remainder
• ✅ Floating-point isn't exact — round for display, use cents for money
• ✅ Choose rounding deliberately: round , floor , ceil , trunc
• ✅ Detect bad values with Number.isNaN and Number.isFinite

Up next: Loops & Iteration — repeat work efficiently with for, while, and for…of! 🔁

Practice quiz

Continue this course

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• Next: Loops & Iteration