Falsy JavaScript

In continuation of my previous blog on the basic JavaScript expressions, this blog is about Falsy and Truthy values in JavaScript.

When presented with a non boolean expression instead of a boolean value, JavaScript uses Truthy or Falsy values instead to determine the result. At first glance, this could be confusing and could lead to errors while scripting.

var a = "someval";
var b;
if (a) { // this is a truthy
    // Block is executed
}
if (b) { // b is undefined and so a falsy
    // Block is not executed
}


The Falsy Values
All expressions, objects and values are Truthy by default. There are exceptions and they are called the Falsy values and are listed below:
  # The empty string; ""
  # null
  # undefined
  # The number 0
  # NaN
  # The boolean false

It gets tricky
If you see the below table, you will note that it gets tricky or rather one should be a little extra careful while dealing with Falsy values.

Expression	Value
Falsy Values
false	false
NaN	false
undefined	false
0	false
""	false
null	false
Truthy Values
Infinity	true
true	true
1	true
"1"	true
"0"	true
Complicating things now
0+0	false
0+"0"	true
"0"+"0"	true
1-1	false
"1"-1	false
"1"-"1"	false
Double Negation
!!0	false
!!1	true
!!"0"	true
!!"1"	true
Logical comparison
null == null	true
null == undefined	true
null == NaN	false
null == ""	false
null == 0	false
null == false	false
undefined == undefined	false
undefined == NaN	false
undefined == ""	false
undefined == 0	false
undefined == false	false
NaN == NaN	false
NaN == ""	false
NaN == 0	false
NaN == false	false
"" == ""	true
"" == 0	true
"" == false	true
0 == 0	true
0 == false	true
false == false	true
Strict Logical comparison
null === null	true
"" === ""	true
"" === 0	false
"" === false	false
0 === 0	true
0 === false	false
false === false	true
Arrays
[ ]	true
![ ]	false
[ ] == true	false

To remember:
# An empty array evaluates to Truthy whereas any boolean operation on an empty array results in a Falsy
# NaN is not equal to anything, not even itself
# Undefined is not equal to anything, not even itself
# Use strict equals (===) to be extra sure while comparing. This compares by type and value
# Avoid using Falsy value and stick to plain  true or  false to be sure of the results.

Now that you have read this far, Congratulations! I expect 0 bugs from your code henceforth :)