Sams Teach Yourself JavaScript in 24 Hours

Understanding Expressions and Operators

An expression is a combination of variables and values that the JavaScript interpreter can evaluate to a single value. The characters that are used to combine these values, such as + and /, are called operators.

Using JavaScript Operators

You've already used some operators, such as the + sign (addition) and the increment and decrement operators. Table 5.1 lists some of the most important operators you can use in JavaScript expressions.

Table 5.1. Common JavaScript Operators

Operator	Description	Example
+	Concatenate (combine) strings	message="this is" + "a test";
+	Add	result = 5 + 7;
-	Subtract	score = score - 1;
*	Multiply	total = quantity * price;
/	Divide	average = sum / 4;
%	Modulo (remainder)	remainder = sum % 4;
++	Increment	TRies++;
--	Decrement	total--;

Along with these, there are also many other operators used in conditional statementsyou'll learn about these in Hour 7, "Controlling Flow with Conditions and Loops."

Operator Precedence

When you use more than one operator in an expression, JavaScript uses rules of operator precedence to decide how to calculate the value. Table 5.1 lists the operators from lowest to highest precedence, and operators with highest precedence are evaluated first. For example, consider this statement:

result = 4 + 5 * 3;

If you try to calculate this result, there are two ways to do it. You could multiply 5 * 3 first and then add 4 (result: 19) or add 4 + 5 first and then multiply by 3 (result: 27). JavaScript solves this dilemma by following the precedence rules: Because multiplication has a higher precedence than addition, it first multiplies 5 * 3 and then adds 4, producing a result of 19.

Sometimes operator precedence doesn't produce the result you want. For example, consider this statement:

result = a + b + c + d / 4;

This is an attempt to average four numbers by adding them all together and then dividing by four. However, because JavaScript gives division a higher precedence than addition, it will divide the d variable by 4 before adding the other numbers, producing an incorrect result.

You can control precedence by using parentheses. Here's the working statement to calculate an average:

result = (a + b + c + d) / 4;

The parentheses ensure that the four variables are added first, and then the sum is divided by four.