JS generate random boolean

Question

You can compare Math.random() to 0.5 directly, as the range of Math.random() is [0, 1) (this means 'in the range 0 to 1 including 0, but not 1'). You can divide the range into [0, 0.5) and [0.5, 1).

var random_boolean = Math.random() < 0.5;

// Example
console.log(Math.random() < 0.1); //10% probability of getting true
console.log(Math.random() < 0.4); //40% probability of getting true
console.log(Math.random() < 0.5); //50% probability of getting true
console.log(Math.random() < 0.8); //80% probability of getting true
console.log(Math.random() < 0.9); //90% probability of getting true

If your project has lodash then you can:

_.sample([true, false])

Alternatively you can use your own sample function (source):

const sample = arr => arr[Math.floor(Math.random() * arr.length)];

For a more cryptographically secure value, you can use crypto.getRandomValues in modern browsers.

Sample:

var randomBool = (function() {
  var a = new Uint8Array(1);
  return function() {
    crypto.getRandomValues(a);
    return a[0] > 127;
  };
})();

var trues = 0;
var falses = 0;
for (var i = 0; i < 255; i++) {
  if (randomBool()) {
    trues++;
  }
  else {
    falses++;
  }
}
document.body.innerText = 'true: ' + trues + ', false: ' + falses;

Note that the crypto object is a DOM API, so it's not available in Node, but there is a similar API for Node.

!Math.round(Math.random());

­­­­­­­­­­­­­­

Impressed a lot by Kelvin's answer I would like to suggest a fairly similar but slightly enhanced solution.

var randomBoolean = Math.random() < 0.5;

This solution is a bit more obvious to read, because the number on the right-hand side of < tells you the probability of getting true rather than of getting false, which is more natural to comprehend. Also < is one symbol shorter than >=;

Potentialy faster solutions...

Bitwise operator approach i just thought of Math.random() + .5 >> 0 or ~~(Math.random() + .5). Here is a performance test to judge for yourself.

let randomBoolean = Math.random() + .5 >> 0;                 //chance of true
const randomBoolean = chance => Math.random() + chance >> 0; //chance of true

The bitwise operators is in this case essentially just the same as using Math.trunc() or Math.floor(), therefore this is also posible Math.trunc(Math.random() + .5).

let randomBoolean = Math.trunc(Math.random() + .5);
const randomBoolean = chance => Math.trunc(Math.random() + chance);

Other more common solutions

The more common way to get random boolean is probably a comparative approach like Math.random() >= .5 from Kelvin's answer or Math.random() < .5; from Arthur Khazbs's answer, they actualy output true & false, and not 1 & 0.

let randomBoolean = Math.random() >= .5;                 //chance of false
const randomBoolean = chance => Math.random() >= chance; //chance of false

let randomBoolean = Math.random()  < .5;                 //chance of true
const randomBoolean = chance => Math.random() < chance;  //chance of true

The only reason to use the Math.round(Math.random()) approach is simplicity and laziness.