JS Number Methods

JavaScript Number Methods: Parsing, Formatting, & Checks

JavaScript provides built-in methods on the Number object and prototype for parsing strings into numbers, formatting numeric output, and validating integer boundaries.

Number Utilities Architecture

flowchart TD
    NumberUtils["Number Utilities"] --> Parsing["Parsing: parseInt(), parseFloat()"]
    NumberUtils --> Formatting["Formatting: toFixed(), toPrecision(), toString()"]
    NumberUtils --> Validation["Validation: Number.isInteger(), Number.isFinite()"]

Key Number Methods Reference

Practical Code Example

// Demonstrating Number parsing, formatting, and validation methods

// 1. Formatting Numbers for UI Display
const rawPrice = 1299.9482;
console.log(`Fixed 2 decimals: $${rawPrice.toFixed(2)}`); // "$1299.95"
console.log(`Precision (4 digits): ${rawPrice.toPrecision(4)}`); // "1300"

// 2. String Parsing with parseInt and parseFloat
const pxString = "120px";
const parsedInt = parseInt(pxString, 10); // Always specify radix 10!
console.log(`Parsed Int from '120px': ${parsedInt}`); // 120

const floatString = "99.95 USD";
const parsedFloat = parseFloat(floatString);
console.log(`Parsed Float: ${parsedFloat}`); // 99.95

// 3. Internationalized Currency Formatting (Intl.NumberFormat)
const currencyFormatter = new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD"
});
console.log(`Formatted USD: ${currencyFormatter.format(rawPrice)}`); // "$1,299.95"

// 4. Number Validation Checks
console.log(`Number.isInteger(42): ${Number.isInteger(42)}`);        // true
console.log(`Number.isInteger(42.5): ${Number.isInteger(42.5)}`);    // false

Best Practices & Gotchas

• Always Supply Radix to parseInt(): Always pass 10 as the second parameter (parseInt(str, 10)) to avoid unintended octal or hex parsing.
• toFixed() Returns a String: Remember that toFixed() returns a string, not a number. Wrap in Number() if further math operations are needed.
• Prefer Number.isFinite(): Use Number.isFinite() over global isFinite() to avoid implicit type coercion.

Self-Check Challenge

Parse the hex string "FF" into a base-10 decimal integer using parseInt().