JavaScript Number parseInt() Method

What You’ll Learn

Number.parseInt() is a static method on Number (same API as MDN Number.parseInt). Call Number.parseInt(string, radix) — not on a number instance. It turns text into an integer, optionally in base 2–36. This tutorial covers syntax, radix, junk suffixes, five examples, and try-it labs.

Introduction

Form fields, CSS values, and URLs often store numbers as text — "42", "42px", or even "ff" in hex. Number.parseInt() reads the leading integer from that text.

It is the same function as the global parseInt ((Number.parseInt === parseInt). The Number. form keeps number helpers together on one object.

This page is part of JavaScript Number Methods. Related tools include toString(radix) for the reverse direction, and Number.isNaN() to check failed parses.

Understanding the parseInt() Method

Call Number.parseInt(string) or Number.parseInt(string, radix). The engine:

• Coerces the first argument to a string and skips leading whitespace.
• Reads a sign and digits that are valid for the chosen radix.
• Stops at the first invalid character (so "42px" still yields 42).
• Returns NaN if no valid digit is found (or if radix is out of range).

📝 Syntax

General form of the static method Number.parseInt:

Number.parseInt(string)
Number.parseInt(string, radix)

Parameters

• string (any) — value to parse (coerced to string). Leading whitespace is ignored.
• radix (number, optional) — integer from 2 to 36. If omitted or 0, assume 10, unless the string starts with 0x/0X (then 16).

Return value

An integer number, or NaN:

Exceptions

• None for normal calls — bad input yields NaN rather than a thrown error.

Common patterns

Number.parseInt("42");           // 42
Number.parseInt("42px");         // 42
Number.parseInt("1010", 2);      // 10
Number.parseInt("ff", 16);       // 255
Number.parseInt("hello");        // NaN
Number.parseInt === parseInt;    // true

Examples Gallery

Examples follow MDN / parseInt patterns. Use View Output or Try It Yourself for each case.

Number.parseInt("42");  // 42

42

Number.parseInt("42px");  // 42

42

Number.parseInt("1010", 2);  // 10

10

Number.parseInt("ff", 16);  // 255

255

Number.parseInt("hello");  // NaN

NaN

🚀 Common Use Cases

• Form inputs — turn typed text into integers for counts and ages.
• CSS / DOM sizes — pull the number out of "16px".
• Hex colors / bytes — Number.parseInt(channel, 16).
• Binary flags — parse bit strings with radix 2.
• URL / query values — convert page numbers from strings.
• Not for floats — use Number.parseFloat when decimals matter.

📝 Notes

• This is a static method — use Number.parseInt(s, radix) only.
• Number.parseInt === parseInt — same built-in function.
• Prefer an explicit radix (often 10) to avoid surprises.
• Truncates toward zero for decimal points: Number.parseInt("15.9") is 15.
• Radix outside 2–36 (except the special 0 case) yields NaN.
• Check failures with Number.isNaN(), not === NaN.

💡 Best Practices