Speaking JavaScript

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]
'1,23'
> String([1, 2])
'1,2'
> String([3])
'3'

Note

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, ...