Understanding JavaScript Truthy and Falsy

Question

Can someone please explain JavaScript Truthy and Falsy, using the below sample data. I have read other threads but still confused.

var a = 0;

var a = 10 ==          


        

Answer 1

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.

Answer 2

FALSY

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

note : Empty array ([]) is not falsy

TRUTHY

• Everything that is not FALSY

Answer 3

From my understanding, I believe that var a = 1; is the only truthy and the rest are falsy’s - is this correct?

No.

1. var a = 0;

   Number zero is falsy. However, note that the string zero "0" is truthy.

2. var a = 10 == 5;

   This is same as var a = (10 == 5);, so this is falsy.

3. var a = 1;

   var a = -1;

   Any non-zero number including negative numbers is truthy.

Quoting from MDN

In JavaScript, a truthy value is a value that translates to true when evaluated in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, "", null, undefined, and NaN).

List of falsy values in JavaScript:^{From MDN}

1. false
2. null
3. undefined
4. 0
5. NaN
6. '', "", ``(Empty template string)
7. document.all
8. 0n: BigInt
9. -0

Answer 4

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: https://developer.mozilla.org/en-US/docs/Glossary/Falsy

Answer 5

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 

Answer 6

Truthy -> Value that resolve to true in boolean context

Falsy -> Value that resolve to false in boolean context

For better understanding, falsy values is given below.

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