Chapter 9. Operators
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Chapter 9. Operators

This chapter gives an overview of operators.

Operators and Objects

All operators coerce (as discussed in Type Coercion) their operands to appropriate types. Most operators only work with primitive values (e.g., arithmetic operators and comparison operators). That means that objects are converted to primitives before anything is done with them. One example where that is unfortunate is the plus operator, which many languages use for array concatenation. That’s not so with JavaScript, however, where this operator converts arrays to strings and appends them:

> [1, 2] + [3]
> String([1, 2])
> String([3])


There is no way to overload or customize operators in JavaScript, not even equality.

Assignment Operators

There are several ways to use the plain assignment operator:

x = value
Assigns to a variable x that has previously been declared
var x = value
Combines a variable declaration with an assignment
obj.propKey = value
Sets a property
obj['propKey'] = value
Sets a property
arr[index] = value
Sets an array element[10]

An assignment is an expression that evaluates to the assigned value. That allows you to chain assignments. For example, the following statement assigns 0 to both y and x:

x = y = 0;

Compound Assignment Operators

A compound assignment operator is written as op=, where op is one of several binary operators and = is the assignment operator. The following two expressions are equivalent:

myvar op= value
myvar = myvar op value

In other words, a compound assignment operator op= applies op to both operands and assigns the result to the first operand. Let’s look at an example of using the plus operator (+) via compound assignment:

> var x = 2;
> x += 3
> x

The following are all compound assignment operators:

Equality Operators: === Versus ==

JavaScript has two ways of determining whether two values are equal:

  • Strict equality (===) and strict inequality (!==) consider only values that have the same type to be equal.
  • Normal (or “lenient”) equality (==) and inequality (!=) try to convert values of different types before comparing them as with strict (in)equality.

Lenient equality is problematic in two regards. First, how it performs conversion is confusing. Second, due to the operators being so forgiving, type errors can remain hidden longer.

Always use strict equality and avoid lenient equality. You only need to learn about the latter if you want to know why it should be avoided.

Equality is not customizable. Operators can’t be overloaded in JavaScript, and you can’t customize how equality works. There are some operations where you often need to influence comparison—for example, Array.prototype.sort() (see Sorting and Reversing Elements (Destructive)). That method optionally accepts a callback that performs all comparisons between array elements.

Strict Equality (===, !==)

Values with different types are never strictly equal. If both values have the same type, then the following assertions hold:

Pitfall: NaN

The special number value NaN (see NaN) is not equal to itself:

> NaN === NaN

Thus, you need to use other means to check for it, which are described in Pitfall: checking whether a value is NaN.

Strict inequality (!==)

A strict inequality comparison:

x !== y

is equivalent to the negation of a strict equality comparison:

!(x === y)

Normal (Lenient) Equality (==, !=)

The algorithm for comparing via normal equality works as follows. If both operands have the same type (one of the six specification types—Undefined, Null, Boolean, Number, String, and Object), then compare them via strict equality.

Otherwise, if the operands are:

  1. undefined and null, then they are considered leniently equal:

    > undefined == null
  2. A string and a number, then convert the string to a number and compare both operands via strict equality.
  3. A boolean and a nonboolean, then convert the boolean to a number and compare leniently (again).
  4. An object and a number or a string, then try to convert the object to a primitive (via the algorithm described in Algorithm: ToPrimitive()—Converting a Value to a Primitive) and compare leniently (again).

Otherwise—if none of the aforementioned cases apply—the result of the lenient comparison is false.

Lenient inequality (!=)

An inequality comparison:

x != y

is equivalent to the negation of an equality comparison:

!(x == y)

Pitfall: lenient equality is different from conversion to boolean

Step 3 means that equality and conversion to boolean (see Converting to Boolean) work differently. If converted to boolean, numbers greater than 1 become true (e.g., in if statements). But those numbers are not leniently equal to true. The comments explain how the results were computed:

> 2 == true  // 2 === 1
> 2 == false  // 2 === 0

> 1 == true  // 1 === 1
> 0 == false  // 0 === 0

Similarly, while the empty string is equal to false, not all nonempty strings are equal to true:

> '' == false   // 0 === 0
> '1' == true  // 1 === 1
> '2' == true  // 2 === 1
> 'abc' == true  // NaN === 1

Pitfall: lenient equality and strings

Some of the leniency can be useful, depending on what you want:

> 'abc' == new String('abc')  // 'abc' == 'abc'
> '123' == 123  // 123 === 123

Other cases are problematic, due to how JavaScript converts strings to numbers (see Converting to Number):

> '\n\t123\r ' == 123  // usually not OK
> '' == 0  // 0 === 0

Pitfall: lenient equality and objects

If you compare an object to a nonobject, it is converted to a primitive, which leads to strange results:

> {} == '[object Object]'
> ['123'] == 123
> [] == 0

However, two objects are only equal if they are they same object. That means that you can’t really compare two wrapper objects:

> new Boolean(true) === new Boolean(true)
> new Number(123) === new Number(123)
> new String('abc') == new String('abc')

There Are No Valid Use Cases for ==

You sometimes read about valid use cases for lenient equality (==). This section lists them and points out better alternatives.

Use case: checking for undefined or null

The following comparison ensures that x is neither undefined nor null:

if (x != null) ...

While this is a compact way of writing this check, it confuses beginners, and experts can’t be sure whether it is a typo or not. Thus, if you want to check whether x has a value, use the standard check for truthiness (covered in Truthy and Falsy Values):

if (x) ...

If you want to be more precise, you should perform an explicit check for both values:

if (x !== undefined && x !== null) ...

Use case: working with numbers in strings

If you are not sure whether a value x is a number or a number-as-a-string, you can use checks such as the following:

if (x == 123) ...

The preceding checks whether x is either 123 or '123'. Again, this is very compact, and again, it is better to be explicit:

if (Number(x) === 123) ...

Use case: comparing wrapper instances with primitives

Lenient equals lets you compare primitives with wrapped primitives:

> 'abc' == new String('abc')

There are three reasons against this approach. First, lenient equality does not work between wrapped primitives:

> new String('abc') == new String('abc')

Second, you should avoid wrappers anyway. Third, if you do use them, it is better to be explicit:

if (wrapped.valueOf() === 'abc') ...

Ordering Operators

JavaScript knows the following ordering operators:

  • Less than (<)
  • Less than or equal (<=)
  • Greater than (>)
  • Greater than or equal (>=)

These operators work for numbers and for strings:

> 7 >= 5
> 'apple' < 'orange'

For strings, they are not very useful, because they are case-sensitive and don’t handle features such as accents well (for details, see Comparing Strings).

The Algorithm

You evaluate a comparison:

x < y

by taking the following steps:

  1. Ensure that both operands are primitives. Objects obj are converted to primitives via the internal operation ToPrimitive(obj, Number) (refer to Algorithm: ToPrimitive()—Converting a Value to a Primitive), which calls obj.valueOf() and, possibly, obj.toString() to do so.
  2. If both operands are strings, then compare them by lexicographically comparing the 16-bit code units (see Chapter 24) that represent the JavaScript characters of the string.
  3. Otherwise, convert both operands to numbers and compare them numerically.

The other ordering operators are handled similarly.

The Plus Operator (+)

Roughly, the plus operator examines its operands. If one of them is a string, the other is also converted to a string and both are concatenated:

> 'foo' + 3
> 3 + 'foo'

> 'Colors: ' + [ 'red', 'green', 'blue' ]
'Colors: red,green,blue'

Otherwise, both operands are converted to numbers (see Converting to Number) and added:

> 3 + 1
> 3 + true

That means that the order in which you evaluate matters:

> 'foo' + (1 + 2)
> ('foo' + 1) + 2

The Algorithm

You evaluate an addition:

value1 + value2

by taking the following steps:

  1. Ensure that both operands are primitives. Objects obj are converted to primitives via the internal operation ToPrimitive(obj) (refer to Algorithm: ToPrimitive()—Converting a Value to a Primitive), which calls obj.valueOf() and, possibly, obj.toString() to do so. For dates, obj.toString() is called first.
  2. If either operand is a string, then convert both to strings and return the concatenation of the results.
  3. Otherwise, convert both operands to numbers and return the sum of the results.

Operators for Booleans and Numbers

The following operators only have operands of a single type and also produce results of that type. They are covered elsewhere.

Boolean operators:

Number operators:

Special Operators

Here we will review special operators, namely the conditional, comma, and void operators.

The Conditional Operator ( ? : )

The conditional operator is an expression:

«condition» ? «if_true» : «if_false»

If the condition is true, the result is if_true; otherwise, the result is if_false. For example:

var x = (obj ? obj.prop : null);

The parentheses around the operator are not needed, but they make it easier to read.

The Comma Operator

«left» , «right»

The comma operator evaluates both operands and returns the result of right. Roughly, it does for expressions what the semicolon does for statements.

This example demonstrates that the second operand becomes the result of the operator:

> 123, 'abc'

This example demonstrates that both operands are evaluated:

> var x = 0;
> var y = (x++, 10);

> x
> y

The comma operator is confusing. It’s better to not be clever and to write two separate statements whenever you can.

The void Operator

The syntax for the void operator is:

void «expr»

which evaluates expr and returns undefined. Here are some examples:

> void 0
> void (0)

> void 4+7  // same as (void 4)+7
> void (4+7)

> var x;
> x = 3
> void (x = 5)
> x

Thus, if you implement void as a function, it looks as follows:

function myVoid(expr) {
    return undefined;

The void operator is associated closely with its operand, so use parentheses as necessary. For example, void 4+7 binds as (void 4)+7.

Why does JavaScript have a void operator?

According to JavaScript creator Brendan Eich, he added it to the language to help with javascript: links (one of the aforementioned use cases):

I added the void operator to JS before Netscape 2 shipped to make it easy to discard any non-undefined value in a javascript: URL.[12]

Categorizing Values via typeof and instanceof

If you want to categorize a value, you unfortunately have to distinguish between primitives and objects (refer back to Chapter 8) in JavaScript:

  • The typeof operator distinguishes primitives from objects and determines the types of primitives.
  • The instanceof operator determines whether an object is an instance of a given constructor. Consult Chapter 17 for more information on object-oriented programming in JavaScript.

typeof: Categorizing Primitives

The typeof operator:

typeof «value»

returns a string describing what kind of value value is. Here are some examples:

> typeof undefined
> typeof 'abc'
> typeof {}
> typeof []

typeof is used to distinguish primitives and objects and to categorize primitives (which cannot be handled by instanceof). Unfortunately, the results of this operator are not completely logical and only loosely correspond to the types of the ECMAScript specification (which are explained in JavaScript’s Types):

Operand Result

undefined, undeclared variable




Boolean value


Number value


String value




All other normal values


(Engine-created value)

JavaScript engines are allowed to create values for whom typeof returns arbitrary strings (different from all results listed in this table).

Pitfall: typeof null

Unfortunately, typeof null is 'object'. This is considered a bug (null is not a member of the internal type Object), but it can’t be fixed, because doing so would break existing code. You thus have to be wary of null. For example, the following function checks whether value is an object:

function isObject(value) {
    return (value !== null
       && (typeof value === 'object'
           || typeof value === 'function'));

Trying it out:

> isObject(123)
> isObject(null)
> isObject({})

The history of typeof null

The first JavaScript engine represented JavaScript values as 32-bit words. The lowest 3 bits of such a word were used as a type tag, to indicate whether the value was an object, an integer, a double, a string, or a boolean (as you can see, even this early engine already stored numbers as integers if possible).

The type tag for objects was 000. In order to represent the value null, the engine used the machine language NULL pointer, a word where all bits are zero. typeof checked the type tag to determine the type of value, which is why it reported null to be an object.[13]

Checking whether a variable exists

The check:

typeof x === 'undefined'

has two use cases:

  1. It determines whether x is undefined.
  2. It determines whether the variable x exists.

Here are examples of both use cases:

> var foo;
> typeof foo === 'undefined'

> typeof undeclaredVariable === 'undefined'

For the first use case, comparing directly with undefined is usually a better choice. However, it doesn’t work for the second use case:

> var foo;
> foo === undefined

> undeclaredVariable === undefined
ReferenceError: undeclaredVariable is not defined

instanceof: Checking Whether an Object Is an Instance of a Given Constructor

The instanceof operator:

«value» instanceof «Constr»

determines whether value has been created by the constructor Constr or a subconstructor. Here are some examples:

> {} instanceof Object
> [] instanceof Array  // constructor of []
> [] instanceof Object  // super-constructor of []

As expected, instanceof is false for the nonvalues undefined and null:

> undefined instanceof Object
> null instanceof Object

But it is also false for all other primitive values:

> 'abc' instanceof Object
> 123 instanceof Object

For details on instanceof, consult The instanceof Operator.

[10] Strictly speaking, setting an array element is a special case of setting a property.

[11] Thanks to Brandon Benvie (@benvie), who told me about using void for IIFEs.

[13] Thanks to Tom Schuster (@evilpies) for pointing me to the source code of the first JavaScript engine.

Next: 10. Booleans