Number Methods

By the end of this lesson, you will be able to format JavaScript numbers for display and convert numeric strings into numbers using methods and helpers such as toFixed and parseInt.

What it is

Number methods are functions that let you turn a number into a formatted string or turn a string into a number. In JavaScript, a primitive number such as 3.14 can call methods like toFixed() because the engine temporarily treats it as a Number object.

toFixed() returns a string with a fixed number of decimal places. parseInt() is a global conversion function, not a method on Number.prototype, but it is commonly used with number formatting and parsing. Related terms include toString(), toPrecision(), toExponential(), Number(), parseFloat(), radix, and NaN.

Why it matters

• It lets you display prices, percentages, and measurements with consistent decimal places.
• It helps convert user input from text fields into usable numbers.
• It prevents accidental string math, such as "19.99" + 5 becoming "19.995".
• It gives you control over rounding and precision for simple output.
• It helps extract integers from mixed text, such as years or IDs.

Syntax or steps

const display = value.toFixed(2);
const integer = parseInt(input, 10);
const numeric = Number(input);

(3.14159).toFixed(2); // "3.14"

Use toFixed(digits) when you have a number and want a string with a set number of decimal places. Use parseInt(string, radix) when you want the leading integer from a string. Use Number(string) when the whole string should become a number.

Example

const price = 19.99;
const taxRate = 0.08;

const priceText = price.toFixed(2);
const taxPercent = (taxRate * 100).toFixed(1);

const input = "  42.75 dollars";
const whole = parseInt(input, 10);
const exact = Number("42.75");
const invalid = Number(input);

console.log(priceText);
console.log(taxPercent);
console.log(whole);
console.log(exact);
console.log(invalid);
console.log(typeof priceText);

price.toFixed(2) formats 19.99 as the string "19.99". (taxRate * 100).toFixed(1) formats the percentage as "8.0". parseInt(input, 10) reads the leading integer 42 and ignores the rest of the text. Number("42.75") converts the full numeric string to the number 42.75. Number(input) returns NaN because the string is not entirely numeric. typeof priceText is "string", not "number".

Common mistakes

• Forgetting that toFixed() returns a string. If you need to do math, convert it back with Number() or parseFloat().
• Omitting the radix in parseInt(). Always pass 10 for decimal integers to avoid unexpected behavior.
• Expecting parseInt() to round decimals. parseInt("0.9", 10) returns 0, not 1.
• Using toFixed() for exact money calculations. JavaScript numbers are binary floating-point values, so rounding can be surprising.

When to use it

Practice

Guided exercise: Format 7.5 as a two-decimal string and convert "123abc" to an integer.

Expected result: "7.50" and 123.

Challenge: What does parseInt("99.999", 10) return, and how would you get 99.999 as a number?

Solution hint: parseInt() truncates at the decimal point, so it returns 99. Use Number("99.999") or parseFloat("99.999") to get 99.999.

Quick check

Question: What does (3.14159).toFixed(2) return?

Answer: It returns the string "3.14".

Summary

Number formatting and conversion are about moving between display text and numeric values. toFixed() is useful for simple decimal display, while parseInt() and Number() help convert strings into numbers.

Always remember the return type: formatting methods usually return strings, and parsing helpers return numbers or NaN.