JavaScript — Numbers

What are Numbers?

JavaScript has one number type that covers both integers and decimals:

const age = 25;        // integer
const price = 9.99;    // decimal
const negative = -10;  // negative
const scientific = 1e6; // 1,000,000 (scientific notation)

Number Methods

const num = 42;

// Conversion
Number("42");       // 42
Number("hello");    // NaN
parseInt("42px");   // 42
parseFloat("3.14"); // 3.14
num.toString();     // "42"

// Checking
Number.isInteger(42);     // true
Number.isInteger(3.14);   // false
Number.isNaN(NaN);        // true
Number.isFinite(42);      // true
Number.isFinite(Infinity); // false

// Formatting
(3.14159).toFixed(2);     // "3.14"
(3.14159).toPrecision(3); // "3.14"
(1000000).toLocaleString(); // "1,000,000"

Math Operations

// Basic arithmetic
5 + 3;    // 8 (addition)
5 - 3;    // 2 (subtraction)
5 * 3;    // 15 (multiplication)
5 / 3;    // 1.666... (division)
5 % 3;    // 2 (modulus/remainder)
5 ** 3;   // 125 (exponentiation)

Math Object

Math.PI;           // 3.141592653589793
Math.E;            // 2.718281828459045

Math.round(4.7);   // 5
Math.ceil(4.3);    // 5
Math.floor(4.7);   // 4

Math.abs(-5);      // 5
Math.max(1, 5, 3); // 5
Math.min(1, 5, 3); // 1

Math.sqrt(16);     // 4
Math.pow(2, 3);    // 8

Math.random();     // 0 to 1 (exclusive)
Math.floor(Math.random() * 10) + 1;  // 1 to 10

Floating Point Quirks

0.1 + 0.2;          // 0.30000000000000004 (not 0.3!)
0.1 + 0.2 === 0.3;  // false

// Fix: compare with tolerance
Math.abs(0.1 + 0.2 - 0.3) < Number.EPSILON;  // true

// Or round before comparing
(0.1 + 0.2).toFixed(1) === "0.3";  // true

Number Conversion

// String to number
+"42";            // 42
Number("42");     // 42
parseInt("42px"); // 42 (ignores trailing text)
parseFloat("3.14em"); // 3.14

// Number to string
String(42);       // "42"
(42).toString();  // "42"
(42).toFixed(2);  // "42.00"

// Check validity
Number.isFinite("hello"); // false
Number.isNaN(NaN);        // true

BigInt

For very large integers beyond Number.MAX_SAFE_INTEGER:

const big = 9007199254740991n;  // note the 'n'
const alsoBig = BigInt(9007199254740991);

// Can't mix BigInt and Number
// 42n + 1;  // TypeError
42n + 1n;    // 43n

Common Patterns

// Random integer between min and max
function randInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

// Clamp a number between bounds
function clamp(value, min, max) {
  return Math.min(Math.max(value, min), max);
}

// Check if even or odd
const isEven = n => n % 2 === 0;

// Round to N decimal places
function roundTo(num, decimals) {
  return Math.round(num * 10 ** decimals) / 10 ** decimals;
}
roundTo(3.14159, 2);  // 3.14

Mini Practice

1. Calculate the area of a circle using Math.PI
2. Generate a random number between 1 and 100
3. Round a decimal to 2 places using toFixed
4. Write a function that returns the larger of two numbers
5. Convert a string price to a formatted currency string

Up Next

Next: Operators →