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 == 5;

var a = 1; 

var a = -1;

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

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

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. ''
7. document.all

Answer 3

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
4. string
5. null
6. undefined
7. NaN