Boolean
Boolean
values can be one of two values: true
or false
, representing the truth value of a logical proposition.
Description
Boolean values are typically produced by relational operators, equality operators, and logical NOT (!
). They can also be produced by functions that represent conditions, such as Array.isArray()
. Note that binary logical operators such as &&
and ||
return the values of the operands, which may or may not be boolean values.
Boolean values are typically used in conditional testing, such as the condition for if...else
and while
statements, the conditional operator (? :
), or the predicate return value of Array.prototype.filter()
.
You would rarely need to explicitly convert something to a boolean value, as JavaScript does this automatically in boolean contexts, so you can use any value as if it's a boolean, based on its truthiness. You are also encouraged to use if (condition)
and if (!condition)
instead of if (condition === true)
or if (condition === false)
in your own code so you can take advantage of this convention. However, making sure that values representing conditions are always booleans can help clarify the intent of your code.
// Do this:
// This always returns a boolean value
const isObject = (obj) => !!obj && typeof obj === "object";
// Or this:
const isObject = (obj) => Boolean(obj) && typeof obj === "object";
// Or this:
const isObject = (obj) => obj !== null && typeof obj === "object";
// Instead of this:
// This may return falsy values that are not equal to false
const isObject = (obj) => obj && typeof obj === "object";
Boolean primitives and Boolean objects
For converting non-boolean values to boolean, use Boolean
as a function or use the double NOT operator. Do not use the Boolean()
constructor with new
.
const good = Boolean(expression);
const good2 = !!expression;
const bad = new Boolean(expression); // don't use this!
This is because all objects, including a Boolean
object whose wrapped value is false
, are truthy and evaluate to true
in places such as conditional statements. (See also the boolean coercion section below.)
if (new Boolean(true)) {
console.log("This log is printed.");
}
if (new Boolean(false)) {
console.log("This log is ALSO printed.");
}
const myFalse = new Boolean(false); // myFalse is a Boolean object (not the primitive value false)
const g = Boolean(myFalse); // g is true
const myString = new String("Hello"); // myString is a String object
const s = Boolean(myString); // s is true
Warning: You should rarely find yourself using Boolean
as a constructor.
Boolean coercion
Many built-in operations that expect booleans first coerce their arguments to booleans. The operation can be summarized as follows:
- Booleans are returned as-is.
undefined
turns intofalse
.null
turns intofalse
.0
,-0
, andNaN
turn intofalse
; other numbers turn intotrue
.0n
turns intofalse
; other BigInts turn intotrue
.- The empty string
""
turns intofalse
; other strings turn intotrue
. - Symbols turn into
true
. - All objects become
true
.
Note: A legacy behavior makes document.all
return false
when used as a boolean, despite it being an object. This property is legacy and non-standard and should not be used.
Note: Unlike other type conversions like string coercion or number coercion, boolean coercion does not attempt to convert objects to primitives by calling user methods.
In other words, there are only a handful of values that get coerced to false
— these are called falsy values. All other values are called truthy values. A value's truthiness is important when used with logical operators, conditional statements, or any boolean context.
There are two ways to achieve the same effect in JavaScript.
- Double NOT:
!!x
negatesx
twice, which convertsx
to a boolean using the same algorithm as above. - The
Boolean()
function:Boolean(x)
uses the same algorithm as above to convertx
.
Note that truthiness is not the same as being loosely equal to true
or false
.
if ([]) {
console.log("[] is truthy");
}
if ([] == false) {
console.log("[] == false");
}
// [] is truthy
// [] == false
[]
is truthy, but it's also loosely equal to false
. It's truthy, because all objects are truthy. However, when comparing with false
, which is a primitive, []
is also converted to a primitive, which is ""
via Array.prototype.toString()
. Comparing strings and booleans results in both being converted to numbers, and they both become 0
, so [] == false
is true
. In general, falsiness and == false
differ in the following cases:
NaN
,undefined
, andnull
are falsy but not loosely equal tofalse
."0"
(and other string literals that are not""
but get coerced to 0) is truthy but loosely equal tofalse
.- Objects are always truthy, but their primitive representation may be loosely equal to
false
.
Truthy values are even more unlikely to be loosely equal to true
. All values are either truthy or falsy, but most values are loosely equal to neither true
nor false
.
Constructor
Boolean()
-
Creates
Boolean
objects. When called as a function, it returns primitive values of type Boolean.
Instance properties
These properties are defined on Boolean.prototype
and shared by all Boolean
instances.
Boolean.prototype.constructor
-
The constructor function that created the instance object. For
Boolean
instances, the initial value is theBoolean
constructor.
Instance methods
Boolean.prototype.toString()
-
Returns a string of either
true
orfalse
depending upon the value of the object. Overrides theObject.prototype.toString()
method. Boolean.prototype.valueOf()
-
Returns the primitive value of the
Boolean
object. Overrides theObject.prototype.valueOf()
method.
Examples
Creating false values
const bNoParam = Boolean();
const bZero = Boolean(0);
const bNull = Boolean(null);
const bEmptyString = Boolean("");
const bfalse = Boolean(false);
Creating true values
const btrue = Boolean(true);
const btrueString = Boolean("true");
const bfalseString = Boolean("false");
const bSuLin = Boolean("Su Lin");
const bArrayProto = Boolean([]);
const bObjProto = Boolean({});
Specifications
Specification |
---|
ECMAScript Language Specification # sec-boolean-objects |
Browser compatibility
BCD tables only load in the browser
See also
- Boolean
- Boolean primitives
- Boolean data type on Wikipedia