5.1 Expressions

An expression is a phrase of JavaScript that a JavaScript interpreter can evaluate to produce a value. The simplest expressions are literals or variable names, like these:

1.7                       // A numeric literal
"JavaScript is fun!"      // A string literal
true                      // A boolean literal
null                      // The literal null value
/java/                    // A regular expression literal
{ x:2, y:2 }              // An object literal
[2,3,5,7,11,13,17,19]     // An array literal
function(x){return x*x;}  // A function literal
i                         // The variable i
sum                       // The variable sum 

The value of a literal expression is simply the literal value itself. The value of a variable expression is the value that the variable contains or refers to.

These expressions are not particularly interesting. More complex (and interesting) expressions can be created by combining simple expressions. For example, we saw that 1.7 is an expression and i is an expression. The following is also an expression:

i + 1.7 

The value of this expression is determined by adding the values of the two simpler expressions. The + in this example is an operator that is used to combine two expressions into a more complex expression. Another operator is -, which is used to combine expressions by subtraction. For example:

(i + 1.7) - sum 

This expression uses the - operator to subtract the value of the sum variable from the value of our previous expression, i + 1.7. JavaScript supports a number of other operators besides + and -, as you'll see in the next section.