Number Methods & Parsing

Syntax

num.toFixed(digits)
parseInt(string)
parseFloat(string)

Examples

const price = 19.9899;

console.log(price.toFixed(2)); // "19.99" - returns a STRING
console.log(price.toFixed(0)); // "20"

const whole = 5;
console.log(whole.toFixed(2)); // "5.00"

console.log(parseInt("42px"));     // 42 - stops at the first non-numeric character
console.log(parseInt("3.14"));     // 3 - parseInt ignores decimals
console.log(parseFloat("3.14px")); // 3.14
console.log(parseInt("abc"));      // NaN

// Always specify the radix for parseInt to avoid ambiguity
console.log(parseInt("08", 10)); // 8

console.log(Number.isInteger(5));    // true
console.log(Number.isInteger(5.5));  // false

console.log(Number.isNaN(NaN));      // true
console.log(Number.isNaN("hello"));  // false - unlike global isNaN(), does not coerce first
console.log(isNaN("hello"));         // true - global isNaN coerces "hello" to NaN first, which is misleading

Best practices

• Always specify the radix (base) as the second argument to parseInt(), like parseInt(str, 10), to avoid ambiguity with leading zeros
• Remember toFixed() returns a string, not a number - convert back with Number() if further math is needed
• Prefer Number.isNaN() over the global isNaN(), since the global version coerces its argument first and can give misleading results
• Use Number.isInteger() rather than % 1 === 0 checks for clearer, more explicit integer validation

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation