# How to convert binary to decimal in JavaScript

A binary number is a numeral system with a base of 2, using only two symbols (0 and 1) to represent all values. In contrast, the decimal system, which is the most common numeral system used by people, has a base of 10 and uses ten symbols (0 through 9) to represent values.

Given a binary number:

bin = $b_{n-1}b_{n-2}...b_2b_1b_0$

In this number, $n$ represents the number of bits in the binary number, and the value of $b_i$ is either $0$ or $1$ for each digit. You can use the following equation to calculate decimal number manually:

dec = $b_{n-1} * 2^{n-1} + b_{n-2} * 2^{n-2} + ... + b_2 * 2^2 + b_1 * 2^1 + b_0 * 2^0$

For example, if you have the binary number $1101$:

dec = $1 * 2^3 + 1 * 2^2 + 0 * 2^1 + 1 * 2^0$ = $8 + 4 + 0 + 1$ = 13

## Using parseInt function

You can also use the 0b prefix to create a binary numeric literal in JavaScript. This means that you can write a binary number directly in your code, without using quotes.

let bin = 0b101010;
console.log(bin); // 42
console.log(typeof bin); // "number"

A binary string is a string that consists of only "0" and "1" characters. In JavaScript, you can use the parseInt() function to convert a binary string to a decimal number, or the toString() method to convert a decimal number to a binary string.

// Convert binary string to decimal number
let bin = "101010";
let dec = parseInt(bin, 2); // Specify radix 2 for binary
console.log(dec); // 42

// Convert decimal number to binary string
let num = 42;
let sbin = num.toString(2); // Specify radix 2 for binary
console.log(sbin); // "101010"

The parseInt function takes a string as the first argument and a radix as the second argument, and returns an integer in decimal. The radix is the base of the number system that the string represents. For example, binary is base 2, decimal is base 10, hexadecimal is base 16, and so on.

You canβt use the 0b prefix in a binary string when using parseInt because the parseInt function does not recognize it as a valid radix prefix. The only prefix that parseInt recognizes is 0x or 0X for hexadecimal values. Everything else is parsed as a decimal value if the radix is missing. If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point.

let bin = "0b101010";
let dec = parseInt(bin, 2); // Specify radix 2 for binary
console.log(dec); // 0

## Using Number constructor

The Number() constructor in JavaScript is a function that can create Number objects or convert values to numbers. It can handle binary numbers and binary strings in different ways, depending on how you use it, but it does not accept a second argument for the radix.

The Number() constructor creates Number objects. When called as a function, it returns primitive values of type Number.

If you pass a binary number using 0b prefix to the Number constructor, it will return the same number as a decimal number.

let bin = 0b101010; // 0b101010 is 42 in decimal
let dec = Number(bin); // No radix specified
console.log(dec); // 42
console.log(typeof dec); // "number"

Also work with binary string using 0b prefix:

let bin = "0b101010"; // 0b101010 is 42 in decimal
let dec = Number(bin);
console.log(dec); // 42
console.log(typeof dec); // "number"

Without 0b prefix, binary string passed to Number constructor will be used as is decimal:

let bin = "101010";
let dec = Number(bin);
console.log(dec); // 101010
console.log(typeof dec); // "number"