Understanding Javascript Truthy and Falsy

There's a simple way to check, which you can use now and forever:

function truthyOrFalsy(a) {
    return a ? "truthy" : "falsy";
}

To wit:

> truthyOrFalsy(0)
"falsy"
> truthyOrFalsy(10 == 5)
"falsy"
> truthyOrFalsy(1)
"truthy"
> truthyOrFalsy(-1)
"truthy"

Also see a list of all falsey values in JavaScript.

Truthy -> Value that resolve to true in boolean context

Falsy -> Value that resolve to false in boolean context

1. false
2. 0
3. empty string
4. null
5. undefined
6. NaN

FALSY

• false
• 0 (zero)
• "", '', `` (empty strings)
• null
• undefined
• NaN (not a number)

note : Empty array ([]) is not falsy

TRUTHY

• Everything that is not FALSY

The below answer might help someone.

As well as a type, each value also has an inherent Boolean value, generally known as either truthy or falsy. Some of the rules are a little bizarre, so understanding the concepts and effect on comparison helps when debugging JavaScript applications.

The following values are always falsy:

• false
• 0 (zero)
• -0 (minus zero)
• 0n (BigInt zero)
• '', "", `` (empty string)
• null
• undefined
• NaN

Everything else is truthy. That includes:

• '0' (a string containing a single zero)
• 'false' (a string containing the text “false”)
• [] (an empty array)
• {} (an empty object)
• function(){} (an “empty” function)

A single value can therefore be used within conditions. For example:

if (value) { // value is truthy } else { // value is falsy // it could be false, 0, '', null, undefined or NaN }

In JavaScript, && and || don't always produce a boolean value. Both operators always return the value of one of their operand expressions. Using the double negation !! or the Boolean function, "truthy" and "falsy" values can be converted to proper booleans.

true && true =>
true

true && false =>
false

true && 'rahul626'=>
"rahul626"

true && 'i am testing Truthy' && ' upvote it'=>
" upvote it"

Try, tried on console

Well described here

one more check version:

function truthyOrFalsy(a) {
    return (a && "truthy") || "falsy";
}

In short there are only 6 types of falsy values: You can use this snippet to test them:

function isTruthy(val){
    if(val){
        console.log(val + ' is Truthy');
    }else{
        console.log(val + ' is falsy');
    }
}
    

// all below are truthy
isTruthy (true)
isTruthy ({})
isTruthy ([])
isTruthy (42)
isTruthy ("0")
isTruthy ("false")
isTruthy (new Date())
isTruthy (-42)
isTruthy (12n)
isTruthy (3.14)
isTruthy (-3.14)
isTruthy (Infinity)
isTruthy (-Infinity)

//all below are falsy
isTruthy(0);
isTruthy("");
isTruthy(false);
isTruthy(NaN);
isTruthy(null);
isTruthy(undefined);

Refer this site for details:

Easy way to check Falsy Value and True value

function truthyOrFalsy(val){
  if(val){
    console.log (`${val} is truthy`);
  } else{
    console.log (`${val} is falsy`);
  }   
}

Check all FALSY value:

truthyOrFalsy(false);      //Output: false is falsy
truthyOrFalsy(null);       //Output: null is falsy
truthyOrFalsy(0);          //Output: 0 is falsy
truthyOrFalsy('');         //Output:  is falsy  [blank refers to '']
truthyOrFalsy(NaN);        //Output: NaN is falsy
truthyOrFalsy(undefined);  //Output: undefined is falsy

Please note that undefined is not explicitly used to set as value. Some common scenarios will create undefined:

• Parameter defined in function but not passed argument in callback function.
• If nothing returns in function
• If accessing to an object property/method which is not defined
• If accessing to an array element which is not defined

function add(num1, num2){   
    console.log(num1, num2);    
}
const result = add(44);
console.log(result);
//Output: 44 undefined
//        undefined

const car = {color:"Blue", price: 200000};
console.log(car.category);
//Output: undefined
arrColors = ["Blue", "Sky", "Purple"];
console.log(arrColors[5]);
//Output: undefined

Check all TRUTHY values

All values are truthy unless they are defined as falsy.

Although ' ', '0', -1, [] could be enlisted to be checked.

truthyOrFalsy(' ');      //Output: is truty     [blank refers to space inside 
                         //                       quote ]
truthyOrFalsy('0');       //Output: 0 is truty 
truthyOrFalsy([]);          //Output: is truty  [blank refers to an empty array]
truthyOrFalsy(-1);         //Output: -1 is truty 

Another way to evaluate whether something is truthy or falsy that I like to use is

function truthyOrFalsy(a) {
  return !!a;
}