Expressions

Expressions

An expression is any valid set of literals, variables, operators, and expressions that evaluates to a single value. The value may be a number, a string, or a logical value. Conceptually, there are two types of expressions: those that assign a value to a variable, and those that simply have a value. For example, the expression

x = 7

is an expression that assigns x the value 7. This expression itself evaluates to 7. Such expressions use assignment operators. On the other hand, the expression

3 + 4

simply evaluates to 7; it does not perform an assignment. The operators used in such expressions are referred to simply as operators.

JavaScript has the following kinds of expressions:

• Arithmetic: evaluates to a number, for example
• String: evaluates to a character string, for example "Fred" or "234"
• Logical: evaluates to true or false

The special keyword null denotes a null value. In contrast, variables that have not been assigned a value are undefined, and cannot be used without a run-time error.

Conditional Expressions

A conditional expression can have one of two values based on a condition. The syntax is

(condition) ? val1 : val2

If condition is true, the expression has the value of val1, Otherwise it has the value of val2. You can use a conditional expression anywhere you would use a standard expression.

For example,

status = (age >= 18) ? "adult" : "minor"

This statement assigns the value "adult" to the variable status if age is eighteen or greater. Otherwise, it assigns the value "minor" to status.