JavaScript
Booleans
A JavaScript
Boolean represents one of two values: true or false.
Boolean
Values
Very often,
in programming, you will need a data type that can only have one of two values,
like
YES / NO
ON / OFF
TRUE / FALSE
For this,
JavaScript has a Boolean data type. It can only take the values true or false.
The
Boolean() Function
You can use
the Boolean() function to find out if an expression (or a variable) is true:
Example
Boolean(10
> 9) // returns true
»
Or even
easier:
Example
(10
> 9) // also returns true
10
> 9 // also returns
true
»
Comparisons
and Conditions
The chapter
JS Comparisons gives a full overview of comparison operators.
The chapter
JS Conditions gives a full overview of conditional statements.
Here are
some examples:
Operator Description Example
== equal to
if (day == "Monday")
> greater than if (salary > 9000)
< less than
if (age < 18)
The Boolean
value of an expression is the fundament for JavaScript comparisons and
conditions.
Everything
With a "Real" Value is True
Examples
100
3.14
-15
"Hello"
"false"
7
+ 1 + 3.14
5
< 6
»
Everything
Without a "Real" is False
The
Boolean value of 0 (zero) is false:
var
x = 0;
Boolean(x); // returns false
»
The
Boolean value of -0 (minus zero) is false:
var
x = -0;
Boolean(x); // returns false
»
The
Boolean value of "" (empty string) is false:
var
x = "";
Boolean(x); // returns false
»
The
Boolean value of undefined is false:
var
x;
Boolean(x); // returns false
»
The
Boolean value of null is false:
var
x = null;
Boolean(x); // returns false
»
The
Boolean value of false is (you guessed it) false:
var
x = false;
Boolean(x); // returns false
»
The
Boolean value of NaN is false:
var
x = 10 / "H";
Boolean(x); // returns false
»
Booleans Can
be Objects
Normally
JavaScript booleans are primitive values created from literals: var x = false
But booleans
can also be defined as objects with the keyword new: var y = new Boolean(false)
Example
var
x = false;
var
y = new Boolean(false);
//
typeof x returns boolean
//
typeof y returns object
Do not
create Boolean objects. It slows down execution speed.
The new
keyword complicates the code. This can produce some unexpected results:
When using
the == operator, equal booleans are equal:
Example
var
x = false;
var
y = new Boolean(false);
//
(x == y) is true because x and y have equal values
»
When using
the === operator, equal booleans are not equal, because the === operator
expects equality in both type and value.
Example
var
x = false;
var
y = new Boolean(false);
//
(x === y) is false because x and y have different types
»
Or even
worse. Objects cannot be compared:
Example
var
x = new Boolean(false);
var
y = new Boolean(false);
// (x == y)
is false because objects cannot be compared
»
Note the
difference between (x==y) and (x===y).
Comparing
two JavaScript objects will always return false.
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