Unary Operators

The unary plus operator (+)

The unary plus (+) precedes its operand and evaluates to its operand. It attempts to convert the operand to a number, if it isn't already.

Syntax:

+expression

Returns:

  • a Number.

Description

The unary plus (+) operator is the fastest (and preferred) method of converting something into a number.

It can convert:

  • string representations of integers (decimal or hexadecimal) and floats.
  • booleans: true, false.
  • null

Values that can't be converted will evaluate to NaN.

Examples:

+42           // 42
+"42"         // 42
+true         // 1
+false        // 0
+null         // 0
+undefined    // NaN
+NaN          // NaN
+"foo"        // NaN
+{}           // NaN
+function(){} // NaN

Note that attempting to convert an array can result in unexpected return values.
In the background, arrays are first converted to their string representations:

[].toString() === '';
[1].toString() === '1';
[1, 2].toString() === '1,2';

The operator then attempts to convert those strings to numbers:

+[]           // 0   ( === +'' )
+[1]          // 1   ( === +'1' )
+[1, 2]       // NaN ( === +'1,2' )

The delete operator

The delete operator deletes a property from an object.

Syntax:

delete object.property

delete object['property']

Returns:

If deletion is successful, or the property did not exist:

  • true

If the property to be deleted is an own non-configurable property (can't be deleted):

  • false in non-strict mode.
  • Throws an error in strict mode

Description

The delete operator does not directly free memory. It can indirectly free memory if the operation means all references to the property are gone.

delete works on an object's properties. If a property with the same name exists on the object's prototype chain, the property will be inherited from the prototype.
delete does not work on variables or function names.

Examples:

// Deleting a property
foo = 1;              // a global variable is a property of `window`: `window.foo`
delete foo;           // true
console.log(foo);     // Uncaught ReferenceError: foo is not defined

// Deleting a variable
var foo = 1;
delete foo;           // false
console.log(foo);     // 1 (Not deleted)

// Deleting a function
function foo(){ };
delete foo;           // false
console.log(foo);     // function foo(){ } (Not deleted)

// Deleting a property
var foo = { bar: "42" };
delete foo.bar;       // true
console.log(foo);     // Object { } (Deleted bar)

// Deleting a property that does not exist
var foo = { };
delete foo.bar;       // true
console.log(foo);     // Object { } (No errors, nothing deleted)

// Deleting a non-configurable property of a predefined object
delete Math.PI;       // false  ()
console.log(Math.PI); // 3.141592653589793 (Not deleted)

The typeof operator

The typeof operator returns the data type of the unevaluated operand as a string.

Syntax:

typeof operand

Returns:

These are the possible return values from typeof:

TypeReturn value
Undefined"undefined"
Null"object"
Boolean"boolean"
Number"number"
String"string"
Symbol (ES6)"symbol"
Function object"function"
document.all"undefined"
Host object (provided by the JS environment)Implementation-dependent
Any other object"object"

The unusual behavior of document.all with the typeof operator is from its former usage to detect legacy browsers. For more information, see Why is document.all defined but typeof document.all returns "undefined"?

Examples:

// returns 'number'
typeof 3.14;
typeof Infinity;
typeof NaN;               // "Not-a-Number" is a "number"

// returns 'string'
typeof "";
typeof "bla";
typeof (typeof 1);        // typeof always returns a string

// returns 'boolean'
typeof true;
typeof false;

// returns 'undefined'
typeof undefined;
typeof declaredButUndefinedVariable;
typeof undeclaredVariable;
typeof void 0;
typeof document.all       // see above

// returns 'function'
typeof function(){};
typeof class C {};
typeof Math.sin;

// returns 'object'
typeof { /*<...>*/ };
typeof null;
typeof /regex/;           // This is also considered an object
typeof [1, 2, 4];         // use Array.isArray or Object.prototype.toString.call.
typeof new Date();
typeof new RegExp();
typeof new Boolean(true); // Don't use!
typeof new Number(1);     // Don't use!
typeof new String("abc"); // Don't use!

// returns 'symbol'
typeof Symbol();
typeof Symbol.iterator;

The void operator

The void operator evaluates the given expression and then returns undefined.

Syntax:

void expression

Returns:

  • undefined

Description

The void operator is often used to obtain the undefined primitive value, by means of writing void 0 or void(0). Note that void is an operator, not a function, so () is not required.

Usually the result of a void expression and undefined can be used interchangeably.
However, in older versions of ECMAScript, window.undefined could be assigned any value, and it is still possible to use undefined as name for function parameters variables inside functions, thus disrupting other code that relies on the value of undefined.
void will always yield the true undefined value though.

void 0 is also commonly used in code minification as a shorter way of writing undefined. In addition, it's probably safer as some other code could've tampered with window.undefined.

Examples:

Returning undefined:

function foo(){
    return void 0;
}
console.log(foo()); // undefined

Changing the value of undefined inside a certain scope:

(function(undefined){
    var str = 'foo';
    console.log(str === undefined); // true
})('foo');

The unary negation operator (-)

The unary negation (-) precedes its operand and negates it, after trying to convert it to number.

Syntax:

-expression

Returns:

  • a Number.

Description

The unary negation (-) can convert the same types / values as the unary plus (+) operator can.

Values that can't be converted will evaluate to NaN (there is no -NaN).

Examples:

-42           // -42
-"42"         // -42
-true         // -1
-false        // -0
-null         // -0
-undefined    // NaN
-NaN          // NaN
-"foo"        // NaN
-{}           // NaN
-function(){} // NaN

Note that attempting to convert an array can result in unexpected return values.
In the background, arrays are first converted to their string representations:

[].toString() === '';
[1].toString() === '1';
[1, 2].toString() === '1,2';

The operator then attempts to convert those strings to numbers:

-[]           // -0  ( === -'' )
-[1]          // -1  ( === -'1' )
-[1, 2]       // NaN ( === -'1,2' )

The bitwise NOT operator (~)

The bitwise NOT (~) performs a NOT operation on each bit in a value.

Syntax:

~expression

Returns:

  • a Number.

Description

The truth table for the NOT operation is:

aNOT a
01
10
1337  (base 10) = 0000010100111001 (base 2)
~1337 (base 10) = 1111101011000110 (base 2) = -1338 (base 10)

A bitwise not on a number results in: -(x + 1).

Examples:

value (base 10)value (base 2)return (base 2)return (base 10)
20000001011111100-3
10000000111111110-2
00000000011111111-1
-111111111000000000
-211111110000000011
-311111100000000102

The logical NOT operator (!)

The logical NOT (!) operator performs logical negation on an expression.

Syntax:

!expression

Returns:

  • a Boolean.

Description

The logical NOT (!) operator performs logical negation on an expression.

Boolean values simply get inverted: !true === false and !false === true.
Non-boolean values get converted to boolean values first, then are negated.

This means that a double logical NOT (!!) can be used to cast any value to a boolean:

!!"FooBar" === true
!!1 === true
!!0 === false

These are all equal to !true:

!'true' === !new Boolean('true');
!'false' === !new Boolean('false');
!'FooBar' === !new Boolean('FooBar');
![] === !new Boolean([]);
!{} === !new Boolean({});

These are all equal to !false:

!0 === !new Boolean(0);
!'' === !new Boolean('');
!NaN === !new Boolean(NaN);
!null === !new Boolean(null);
!undefined === !new Boolean(undefined);

Examples:

!true         // false
!-1           // false
!"-1"         // false
!42           // false
!"42"         // false
!"foo"        // false
!"true"       // false
!"false"      // false
!{}           // false
![]           // false
!function(){} // false

!false        // true
!null         // true
!undefined    // true
!NaN          // true
!0            // true
!""           // true

Overview

Unary operators are operators with only one operand. Unary operators are more efficient than standard JavaScript function calls. Additionally, unary operators can not be overridden and therefore their functionality is guaranteed.

The following unary operators are available:

OperatorOperationExample
deleteThe delete operator deletes a property from an object.
voidThe void operator discards an expression's return value.
typeofThe typeof operator determines the type of a given object.
+The unary plus operator converts its operand to Number type.
-The unary negation operator converts its operand to Number, then negates it.
~Bitwise NOT operator.
!Logical NOT operator.