How do Numbers work in JavaScript? | JavaScript Interview Questions

What You'll Learn

Number representation
Floating point issues
BigInt for large numbers

Number Basics

JavaScript uses 64-bit floating point (IEEE 754) for all numbers.

code.jsJavaScript

// Same type
typeof 42;      // 'number'
typeof 42.5;    // 'number'

// Special values
Infinity
-Infinity
NaN // Not a Number

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

Floating Point Issues

code.jsJavaScript

// Famous precision problem
0.1 + 0.2;  // 0.30000000000000004
0.1 + 0.2 === 0.3; // false

// Solution: Compare with epsilon
Math.abs(0.1 + 0.2 - 0.3) < Number.EPSILON; // true

// Or use integers
const priceInCents = 999;
const dollars = priceInCents / 100; // 9.99

Safe Integer Range

code.jsJavaScript

Number.MAX_SAFE_INTEGER; // 9007199254740991
Number.MIN_SAFE_INTEGER; // -9007199254740991

Number.isSafeInteger(9007199254740991); // true
Number.isSafeInteger(9007199254740992); // false

BigInt

For integers larger than MAX_SAFE_INTEGER.

code.jsJavaScript

const big = 9007199254740992n;
const also = BigInt('9007199254740992');

// Operations
big + 1n;    // 9007199254740993n
big * 2n;    // 18014398509481984n

// Cannot mix with regular numbers
big + 1;     // TypeError
big + BigInt(1); // OK

Number Methods

code.jsJavaScript

const num = 3.14159;

num.toFixed(2);      // '3.14'
num.toPrecision(4);  // '3.142'
num.toString(2);     // '11.001001...' (binary)

parseInt('42px');    // 42
parseFloat('3.14');  // 3.14