A compilation unit contains a set of executable statements. In web browsers, each <script> tag delivers a compilation unit that is compiled and immediately executed. Lacking a linker, JavaScript throws them all together in a common global namespace. There is more on global variables in Appendix A.

When used inside of a function, the var statement defines the function's private variables.

The switch, while, for, and do statements are allowed to have an optional label prefix that interacts with the break statement.

Statements tend to be executed in order from top to bottom. The sequence of execution can be altered by the conditional statements (if and switch), by the looping statements (while, for, and do), by the disruptive statements (break, return, and throw), and by function invocation.

A block is a set of statements wrapped in curly braces. Unlike many other languages, blocks in JavaScript do not create a new scope, so variables should be defined at the top of the function, not in blocks.

The if statement changes the flow of the program based on the value of the expression. The then block is executed if the expression is truthy; otherwise, the optional else branch is taken.

Here are the falsy values:

All other values are truthy, including true, the string 'false', and all objects.

The switch statement performs a multiway branch. It compares the expression for equality with all of the specified cases. The expression can produce a number or a string. When an exact match is found, the statements of the matching case clause are executed. If there is no match, the optional default statements are executed.

A case clause contains one or more case expressions. The case expressions need not be constants. The statement following a clause should be a disruptive statement to prevent fall through into the next case. The break statement can be used to exit from a switch.

The while statement performs a simple loop. If the expression is falsy, then the loop will break. While the expression is truthy, the block will be executed.

The for statement is a more complicated looping statement. It comes in two forms.

The conventional form is controlled by three optional clauses: the initialization, the condition, and the increment. First, the initialization is done, which typically initializes the loop variable. Then, the condition is evaluated. Typically, this tests the loop variable against a completion criterion. If the condition is omitted, then a condition of true is assumed. If the condition is falsy, the loop breaks. Otherwise, the block is executed, then the increment executes, and then the loop repeats with the condition.

The other form (called for in) enumerates the property names (or keys) of an object. On each iteration, another property name string from the object is assigned to the variable.

It is usually necessary to test object.hasOwnProperty(variable) to determine whether the property name is truly a member of the object or was found instead on the prototype chain.

for (myvar in obj) {
    if (obj.hasOwnProperty(myvar)) {
        ...
    }
}

The do statement is like the while statement except that the expression is tested after the block is executed instead of before. That means that the block will always be executed at least once.

The try statement executes a block and catches any exceptions that were thrown by the block. The catch clause defines a new variable that will receive the exception object.

The throw statement raises an exception. If the throw statement is in a try block, then control goes to the catch clause. Otherwise, the function invocation is abandoned, and control goes to the catch clause of the try in the calling function.

The expression is usually an object literal containing a name property and a message property. The catcher of the exception can use that information to determine what to do.

The return statement causes the early return from a function. It can also specify the value to be returned. If a return expression is not specified, then the return value will be undefined.

JavaScript does not allow a line end between the return and the expression.

The break statement causes the exit from a loop statement or a switch statement. It can optionally have a label that will cause an exit from the labeled statement.

JavaScript does not allow a line end between the break and the label.

An expression statement can either assign values to one or more variables or members, invoke a method, delete a property from an object. The = operator is used for assignment. Do not confuse it with the === equality operator. The += operator can add or concatenate.