How do I convert an integer to binary in JavaScript?

The binary in 'convert to binary' can refer to three main things. The positional number system, the binary representation in memory or 32bit bitstrings. (for 64bit bitstrings see Patrick Roberts' answer)

1. Number System

(123456).toString(2) will convert numbers to the base 2 positional numeral system. In this system negative numbers are written with minus signs just like in decimal.

2. Internal Representation

The internal representation of numbers is 64 bit floating point and some limitations are discussed in this answer. There is no easy way to create a bit-string representation of this in javascript nor access specific bits.

3. Masks & Bitwise Operators

MDN has a good overview of how bitwise operators work. Importantly:

Bitwise operators treat their operands as a sequence of 32 bits (zeros and ones)

Before operations are applied the 64 bit floating points numbers are cast to 32 bit signed integers. After they are converted back.

Here is the MDN example code for converting numbers into 32-bit strings.

function createBinaryString (nMask) {
  // nMask must be between -2147483648 and 2147483647
  for (var nFlag = 0, nShifted = nMask, sMask = ""; nFlag < 32;
       nFlag++, sMask += String(nShifted >>> 31), nShifted <<= 1);
  return sMask;
}

createBinaryString(0) //-> "00000000000000000000000000000000"
createBinaryString(123) //-> "00000000000000000000000001111011"
createBinaryString(-1) //-> "11111111111111111111111111111111"
createBinaryString(-1123456) //-> "11111111111011101101101110000000"
createBinaryString(0x7fffffff) //-> "01111111111111111111111111111111"

Try

num.toString(2);

The 2 is the radix and can be any base between 2 and 36

source here

UPDATE:

This will only work for positive numbers, Javascript represents negative binary integers in two's-complement notation. I made this little function which should do the trick, I haven't tested it out properly:

function dec2Bin(dec)
{
    if(dec >= 0) {
        return dec.toString(2);
    }
    else {
        /* Here you could represent the number in 2s compliment but this is not what 
           JS uses as its not sure how many bits are in your number range. There are 
           some suggestions https://stackoverflow.com/questions/10936600/javascript-decimal-to-binary-64-bit 
        */
        return (~dec).toString(2);
    }
}

I had some help from here

A simple way is just...

Number(42).toString(2);

// "101010"

function dec2bin(dec) {
  return (dec >>> 0).toString(2);
}

console.log(dec2bin(1)); // 1
console.log(dec2bin(-1)); // 11111111111111111111111111111111
console.log(dec2bin(256)); // 100000000
console.log(dec2bin(-256)); // 11111111111111111111111100000000

You can use Number.toString(2) function, but it has some problems when representing negative numbers. For example, (-1).toString(2) output is "-1".

To fix this issue, you can use the unsigned right shift bitwise operator (>>>) to coerce your number to an unsigned integer.

If you run (-1 >>> 0).toString(2) you will shift your number 0 bits to the right, which doesn't change the number itself but it will be represented as an unsigned integer. The code above will output "11111111111111111111111111111111" correctly.

This question has further explanation.

-3 >>> 0 (right logical shift) coerces its arguments to unsigned integers, which is why you get the 32-bit two's complement representation of -3.