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Jump Statements

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Another category of JavaScript statements are jump statements. As the name implies, these cause the JavaScript interpreter to jump to a new location in the source code. The break statement makes the interpreter jump to the end of a loop or other statement. continue makes the interpreter skip the rest of the body of a loop and jump back to the top of a loop to begin a new iteration. JavaScript allows statements to be named, or labeled, and the break and continue can identify the target loop or other statement label. The return statement makes the interpreter jump from a function invocation back to the code that invoked it and also supplies the value for the invocation. The throw statement raises, or “throws,” an exception and is designed to work with the try/catch/ finally statement, which establishes a block of exception handling code. This is a complicated kind of jump statement: when an exception is thrown, the interpreter jumps to the nearest enclosing exception handler, which may be in the same function or up the call stack in an invoking function.

break

break;

The break statement, used alone, causes the innermost enclosing loop or switch statement to exit immediately. Its syntax is simple:

Because it causes a loop or switch to exit, this form of the break statement is legal only if it appears inside one of these statements. You’ve already seen examples of the break statement within a switch statement. In loops, it is typically used to exit prematurely when, for whatever reason, there is no longer any need to complete the loop. When a loop has complex termination conditions, it is often easier to implement some of these conditions with break statements rather than trying to express them all in a single loop expression. The following code searches the elements of an array for a particular value. The loop terminates in the normal way when it reaches the end of the array; it terminates with a break statement if it finds what it is looking for in the array:

for(var i = 0; i < a.length; i++) {
  if (a[i] == target) break;
}

continue

The continue statement is similar to the break statement. Instead of exiting a loop, however, continue restarts a loop at the next iteration. The continue statement’s syntax is just as simple as the break statement:

continue;

When the continue statement is executed, the current iteration of the enclosing loop is terminated, and the next iteration begins. This means different things for different types

of loops:

• In a while loop, the specified expression at the beginning of the loop is tested again, and if it’s true, the loop body is executed starting from the top.

• In a do/while loop, execution skips to the bottom of the loop, where the loop condition is tested again before restarting the loop at the top.

• In a for loop, the increment expression is evaluated, and the test expression is tested again to determine if another iteration should be done.

• In a for/in loop, the loop starts over with the next property name being assigned to the specified variable.

Note the difference in behavior of the continue statement in the while and for loops:

a while loop returns directly to its condition, but a for loop first evaluates its increment expression and then returns to its condition. Earlier we considered the behavior of the for loop in terms of an “equivalent” while loop. Because the continue statement behaves differently for these two loops, however, it is not actually possible to perfectly simulate a for loop with a while loop alone.

The following example shows an unlabeled continue statement being used to skip the rest of the current iteration of a loop when an error occurs:

for(i = 0; i < data.length; i++) {
  if (!data[i]) continue; // Can't proceed with undefined data
  total += data[i];
}