Calculator Programming Tutorial

Programming Building Blocks I

Expressions

Side-Effect Operators

All of the operators we have considered so far have shared one property: they have left their operands unchanged. This is generally a good thing: generally, we do not want the statement

a = b + c

to change the values of b and c, only of a.

There are a category of operators, however, which do change the values of their operands. These are known as side-effect operators: they produce a value, and, as an additional effect, change the values of the variables to which they are applied. JavaScript has operators of this category.

JavaScript has four unary operators, pre- and post-increment, and pre- and post-decrement, which both produce a value and change the value of the variable to which they are applied. These operators are denoted by ++ (two plus signs) and -- (two minus signs), placed before or after the variable to be affected, respectively. Both the pre- and post-increment operators increase the value of their operand by 1. Both the pre- and post-decrement operators decrease the value of their operand by 1. The pre-increment and pre-decrement operators produce, as their value, the updated value of their operand; the post-increment and post-decrement produce, as their value, the original value of their operand. This behavior can be summarized in the table below:

These operators are commonly used in indices for counting loops, as described in the Control Flow II section of this tutorial.

Assignment in JavaScript is also an operator, rather than a statement. As such, the expression

a = b

has a value — its value is the value assigned to a. JavaScript also defines a suite of assignment operators that combine another operation with an assignment side-effect. These operators are *=, /=, %=, +=, -=, <<=, >>=, >>>=, &=, ^=, and |=. The effect of each of these is to perform the indicated operation (*, /, %, etc.) on its two operands, assign the result to the left operand, and return the value assigned as the value of the expression.

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Copyright © 2002 Brian Hetrick
Page last updated 13 January 2002.