[javascript] How to check if type is Boolean

How can I check if a variable's type is of type Boolean?

I mean, there are some alternatives such as:

if(jQuery.type(new Boolean()) === jQuery.type(variable))
      //Do something..

But that doesn't seem pretty to me.

Is there a cleaner way to achieve this?

If you want your function can validate boolean objects too, the most efficient solution must be:

function isBoolean(val) {
  return val === false || val === true || val instanceof Boolean;
}

You can create a function that checks the typeof for an argument.

function isBoolean(value) {
  return typeof value === "boolean";
}

In nodejs by using node-boolify we can use isBoolean();

        var isBoolean = require('node-boolify').isBoolean;
        isBoolean(true); //true
        isBoolean('true'); //true
        isBoolean('TRUE'); //false
        isBoolean(1); //true
        isBoolean(2); //false
        isBoolean(false); //true
        isBoolean('false'); //true
        isBoolean('FALSE'); //false
        isBoolean(0); //true
        isBoolean(null); //false
        isBoolean(undefined); //false
        isBoolean(); //false
        isBoolean(''); //false

• The most readable: val === false || val === true.
• Also readable: typeof variable == typeof true.

• The shortest, but not readable at all: !!val === val.

  Explanation:

  • [!!] The double exclamation mark converts the value into a Boolean.
  • [===] The triple equals test for strict equality: both the type (Boolean) and the value have to be the same.
  • If the original value is not a Boolean one, it won't pass the triple equals test. If it is a Boolean variable, it will pass the triple equals test (with both type & value).

  Tests:

  • !!5 === 5 // false
  • !!'test' === 'test' // false
  • let val = new Date(); !!val === val // false
  • !!true === true // true
  • !!false === false // true

There are three "vanilla" ways to check this with or without jQuery.

1. First is to force boolean evaluation by coercion, then check if it's equal to the original value:

   function isBoolean( n ) {
       return !!n === n;
   }
   

2. Doing a simple typeof check:

   function isBoolean( n ) {
       return typeof n === 'boolean';
   }
   

3. Doing a completely overkill and unnecessary instantiation of a class wrapper on a primative:

   function isBoolean( n ) {
       return n instanceof Boolean;
   }
   

The third will only return true if you create a new Boolean class and pass that in.

To elaborate on primitives coercion (as shown in #1), all primitives types can be checked in this way:

• Boolean:

  function isBoolean( n ) {
      return !!n === n;
  }
  

• Number:

  function isNumber( n ) {
      return +n === n;
  }
  

• String:

  function isString( n ) {
      return ''+n === n;
  }
  

if(['true', 'yes', '1'].includes(single_value)) {
    return  true;   
}
else if(['false', 'no', '0'].includes(single_value)) {
    return  false;  
}

if you have a string

With pure JavaScript, you can just simply use typeof and do something like typeof false or typeof true and it will return "boolean"...

But that's not the only way to do that, I'm creating functions below to show different ways you can check for Boolean in JavaScript, also different ways you can do it in some new frameworks, let's start with this one:

function isBoolean(val) {
   return val === false || val === true;
}

Or one-line ES6 way ...

const isBoolean = val => 'boolean' === typeof val;

and call it like!

isBoolean(false); //return true

Also in Underscore source code they check it like this(with the _. at the start of the function name):

isBoolean = function(obj) {
   return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
};

Also in jQuery you can check it like this:

jQuery.type(true); //return "boolean"

In React, if using propTypes, you can check a value to be boolean like this:

MyComponent.propTypes = {
  children: PropTypes.bool.isRequired
};

If using TypeScript, you can use type boolean also:

let isDone: boolean = false;

Also another way to do it, is like converting the value to boolean and see if it's exactly the same still, something like:

const isBoolean = val => !!val === val;

or like:

const isBoolean = val => Boolean(val) === val;

and call it!

isBoolean(false); //return true

It's not recommended using any framework for this as it's really a simple check in JavaScript.

I would go with Lodash: isBoolean checks whether the passed-in variable is either primitive boolean or Boolean wrapper object and so accounts for all cases.

One more decision with es2015 arrow function

const isBoolean = val => typeof val === 'boolean';

You can use pure Javascript to achieve this:

var test = true;
if (typeof test === 'boolean')
   console.log('test is a boolean!');

BENCHMARKING:

All pretty similar...

const { performance } = require('perf_hooks');

const boolyah = true;
var t0 = 0;
var t1 = 0;
const loops = 1000000;
var results = { 1: 0, 2: 0, 3: 0, 4: 0 };

for (i = 0; i < loops; i++) {

    t0 = performance.now();
    boolyah === false || boolyah === true;
    t1 = performance.now();
    results['1'] += t1 - t0;

    t0 = performance.now();
    'boolean' === typeof boolyah;
    t1 = performance.now();
    results['2'] += t1 - t0;

    t0 = performance.now();
    !!boolyah === boolyah;
    t1 = performance.now();
    results['3'] += t1 - t0;

    t0 = performance.now();
    Boolean(boolyah) === boolyah;
    t1 = performance.now();
    results['4'] += t1 - t0;
}

console.log(results);

  // RESULTS
  // '0': 135.09559339284897,
  // '1': 136.38034391403198,
  // '2': 136.29421120882034,
  // '3': 135.1228678226471,
  // '4': 135.11531442403793

Sometimes we need a single way to check it. typeof not working for date etc. So I made it easy by

Date.prototype.getType() { return "date"; }

Also for Number, String, Boolean etc. we often need to check the type in a single way...

Creating functions like isBoolean which contains oneliner typeof v === "boolean" seems very unhandy in long term. i am suprised that almost everyone suggest to create your own function. It seems to be same cancer as extending native prototypes.

• you need to recreate them in every project you are involved
• other developers might have different habits,, or need to check source of your function to see which impementation of check you use, to know what are weak points of your check
• you will be fruustrated when you will try to write one liner in console on site which doesn't belong to your project

just memoize typeof v === "boolean" and that's all. Add a template to your IDE to be able to put it by some three letter shortcut and be happy.

The most reliable way to check type of a variable in JavaScript is the following:

var toType = function(obj) {
  return ({}).toString.call(obj).match(/\s([a-zA-Z]+)/)[1].toLowerCase()
}
toType(new Boolean(true)) // returns "boolean"
toType(true); // returns "boolean"

The reason for this complication is that typeof true returns "boolean" while typeof new Boolean(true) returns "object".

If you just want to check for a primitive value

typeof variable === 'boolean'

If for some strange reason you have booleans created with the constructor, those aren't really booleans but objects containing a primitive boolean value, and one way to check for both primitive booleans and objects created with new Boolean is to do :

function checkBool(bool) {
    return typeof bool === 'boolean' || 
           (typeof bool === 'object' && 
            bool !== null            &&
           typeof bool.valueOf() === 'boolean');
}

function checkBool(bool) {_x000D_
    return typeof bool === 'boolean' || _x000D_
           (typeof bool === 'object' && _x000D_
            bool !== null            &&_x000D_
           typeof bool.valueOf() === 'boolean');_x000D_
}_x000D_
_x000D_
console.log( checkBool( 'string'          )); // false, string_x000D_
console.log( checkBool( {test: 'this'}    )); // false, object_x000D_
console.log( checkBool( null              )); // false, null_x000D_
console.log( checkBool( undefined         )); // false, undefined_x000D_
console.log( checkBool( new Boolean(true) )); // true_x000D_
console.log( checkBool( new Boolean()     )); // true_x000D_
console.log( checkBool( true              )); // true_x000D_
console.log( checkBool( false             )); // true