One easy way to get started at programming is to examine some typical interactions with an interpreter for the JavaScript language. You type a statement, and the interpreter responds by displaying the result of its evaluating that statement.

One kind of statement you might type is an expression statement, which consists of an expression followed by a semicolon. One kind of primitive expression is a number.

(More precisely, the expression that you type consists of the numerals that represent the number in base 10.)

If you ask our script to evaluate the expression statement

486;

by clicking it, it will respond by displaying a JavaScript interpreter with the option to evaluate the statement by pressing a Run button. Click on the primitive expression statement, and see what happens!

Expressions representing numbers may be combined with operators (such as + or *) to form a compound expression that represents the application of a corresponding primitive function to those numbers. For example, evaluate any of the following expression statements by clicking on it:

137 + 349;

486

1000 - 334;

666

5 * 99;

495

10 / 4;

2.5

2.7 + 10;

12.7

Expressions such as these, which contain other expressions as components, are called combinations. Combinations that are formed by an operator symbol in the middle, and operand expressions to the left and right of it, are called operator combinations. The value of an operator combination is obtained by applying the function specified by the operator to the arguments that are the values of the operands.

The convention of placing the operator between the operands is known as infix notation. It follows the mathematical notation that you are most likely familiar with from school and everyday life. As in mathematics, operator combinations can be nested, that is, they can have operands that themselves are operator combinations:

(3 * 5) + (10 - 6);

19

As usual, parentheses are used to group operator combinations in order to avoid ambiguities. JavaScript also follows the usual conventions when parentheses are omitted: multiplication and division bind more strongly than addition and subtraction. For example,

3 * 5 + 10 / 2;

stands for

(3 * 5) + (10 / 2);

We say that * and / have higher precedence than + and -. Sequences of additions and subtractions are read from left to right, as are sequences of multiplications and divisions. Thus,

1 - 5 / 2 * 4 + 3;

stands for

(1 - ((5 / 2) * 4)) + 3;

We say that the operators +, -, * and / are left-associative.

There is no limit (in principle) to the depth of such nesting and to the overall complexity of the expressions that the JavaScript interpreter can evaluate. It is we humans who might get confused by still relatively simple expressions such as

3 * 2 * (3 - 5 + 4) + 27 / 6 * 10;

which the interpreter would readily evaluate to be 57. We can help ourselves by writing such an expression in the form

3 * 2 * (3 - 5 + 4)
+
27 / 6 * 10;

to visually separate the major components of the expression.

Even with complex expressions, the interpreter always operates in the same basic cycle: It reads a statement typed by the user, evaluates the statement, and prints the result. This mode of operation is often expressed by saying that the interpreter runs in a read-evaluate-print loop. Observe in particular that it is not necessary to explicitly instruct the interpreter to print the value of the statement.