JS: falsy expressions | JavaScript
JavaScript falsy expressions
JavaScript falsy expression are those expressions, result as a false boolean value in if condition and ternary ?: operator. Rest of the expression return true.
Falsy Expression means an expression returns false boolean value when prided inifcondition or ternary operators?:
false is falsy
false is obviously falsy in if condition and ternary operator ?::
var x = false;
if(x)
alert('truly');
else
alert('falsy')
falsy
Comparing two false variables
Comparing a variable itself or with another variable having false values, is truly
var x = false;
var y = false;
if(x == y)
alert('truly');
else
alert('falsy')
truly
Comparing two false hard coded
Comparing a variable itself or with another variable having false values, is truly
if(false == false)
alert('truly');
else
alert('falsy')
truly
Empty string "" is false
Empty string "" is treated as false in if condition and ternary operator ?::
var x = "";
if(x)
alert('truly');
else
alert('falsy')
falsy
Comparing two Empty string "" variables
Comparing a variable itself or with another variable having empty "" values, is truly
var x = "";
var y = "";
if(x == y)
alert('truly');
else
alert('falsy')
truly
Comparing two Empty string hard coded
Comparing a variable itself or with another variable having empty string "" values, is truly
if("" == "")
alert('truly');
else
alert('falsy')
truly
Zero 0 is falsy
Zero 0 is treated as false in if condition and ternary operator ?::
var x = 0;
if(x)
alert('truly');
else
alert('falsy')
falsy
Comparing two Zero 0 variables
Comparing a variable itself or with another variable having Zero 0 values, is truly
var x = 0;
var y = 0;
if(x == y)
alert('truly');
else
alert('falsy')
truly
undefined is false
undefined is treated as false in if condition and ternary operator ?::
var x = undefined;
if(x)
alert('truly');
else
alert('falsy')
falsy
Comparing two undefined variables
Comparing a variable itself or with another variable having undefined values, is truly
var x = undefined;
var y = undefined;
if(x == y)
alert('truly');
else
alert('falsy')
truly
Comparing two undefined hard coded
Comparing a variable itself or with another variable having undefined values, is truly
if(undefined == undefined)
alert('truly');
else
alert('falsy')
truly
null is false
null is treated as false in if condition and ternary operator ?::
var x = null;
if(x)
alert('truly');
else
alert('falsy')
falsy
Comparing two null variables
Comparing a variable itself or with another variable having null values, is truly
var x = null;
var y = null;
if(x == y)
alert('truly');
else
alert('falsy')
truly
Comparing two null hard coded
Comparing a variable itself or with another variable having null values, is truly
if(null == null)
alert('truly');
else
alert('falsy')
truly
NaN is false
NaN or Number.NaN is treated as false in if condition and ternary operator ?::
var x = NaN;
if(x)
alert('truly');
else
alert('falsy')
falsy
Comparing two NaN variables
Comparing a variable itself or with another variable having NaN values, is falsy
var x = NaN;
var y = NaN;
if(x == y)
alert('truly');
else
alert('falsy')
falsy
Comparing two NaN hard coded
Comparing a variable itself or with another variable having NaN values, is falsy
if(false == false)
alert('truly');
else
alert('falsy')
falsy
Empty Array [] is true
[] is treated as true in if condition and ternary operator ?::
var x = [];
if(x)
alert('truly');
else
alert('falsy')
truly
Comparing two Empty array variables
Comparing a variable itself having [] values is truly
var x = [];
if(x == x)
alert('truly');
else
alert('falsy')
truly
Comparing a variable with another variable having [] values is falsy
var x = [];
var y = [];
if(x == y)
alert('truly');
else
alert('falsy')
falsy
Comparing two Empty array hard coded
Comparing a variable itself or with another variable having [] values, is falsy
if([] == [])
alert('truly');
else
alert('falsy')
truly
Empty object {} is true
Empty object {} is treated as true in if condition and ternary operator ?::
var x = {};
if(x)
alert('truly');
else
alert('falsy')
truly
Comparing two Empty object variables
Comparing a variable itself having {} values is truly
var x = {};
if(x == x)
alert('truly');
else
alert('falsy')
truly
Comparing a variable with another variable having {} values is falsy
var x = {};
var y = {};
if(x == y)
alert('truly');
else
alert('falsy')
falsy
Comparing two Empty array hard coded
Comparing a variable itself or with another variable having [] values, is falsy
if([] == [])
alert('truly');
else
alert('falsy')
truly
In the the code snipped, we used double-equal sign==instead of recommended tripple-equal sign===to reveal that internal type-casting confusion, that doesn't apply for the falsy expression.
Falsy expressions as string are truly
Falsy expressions like false, 0, null, undefined, NaN etc. are truly i.e. execute the true part of the if condition and ternary operator.
"false" is true
"false" is treated as true because it is a non-empty string:
var x = "flase";
if(x)
alert('truly');
else
alert('falsy')
truly
"0" is true
"0" is treated as true because it is a non-empty string:
var x = "0";
if(x)
alert('truly');
else
alert('falsy')
truly
"undefined" is true
"undefined" is treated as true because it is a non-empty string:
var x = "undefined";
if(x)
alert('truly');
else
alert('falsy')
truly
"null" is true
"null" is treated as true because it is a non-empty string:
var x = "null";
if(x)
alert('truly');
else
alert('falsy')
truly
"NaN" is true
"NaN" is treated as true because it is a non-empty string:
var x = "0";
if(x)
alert('truly');
else
alert('falsy')
truly
As we have seen that "false", 0, "", null, undefined, NaN, NaN == NaN execute the false the false block of the if condition and ternary operator ?: hence they are falsy expressions.