Summary

In this chapter, we have concluded our look at the core of the JavaScript language and its syntax. Everything from now on builds on these foundations, and with the less interesting syntax under our belts, we can move on to more interesting things in the remainder of the book.

The chapter looked at the following:

• Decision making with the if and switch statements. The ability to make decisions is essentially what gives the code its "intelligence." Based on whether a condition is true or false, we can decide on a course of action to follow.

• Comparison operators. The comparison operators compare the value on the left of the operator (left-hand side, LHS) with the value on the right of the operator (right-hand side, RHS) and return a Boolean value. The main comparison operators are

  • ==   Is the LHS equal to the RHS?

  • !=   Is the LHS not equal to the RHS?

  • <=   Is the LHS less than or equal to the RHS?

  • >=   Is the LHS greater than or equal to the RHS?

  • <   Is the LHS less than the RHS?

  • >   Is the LHS greater than the RHS?

• The if statement. Using the if statement, we can choose to execute a block of code (defined by being in curly braces) when a condition is true. The if statement has a test condition, specified in parentheses. If this evaluates to true, the code after the if statement will execute.

• The else statement. If we want code to execute when the if statement is false, we can use the else statement that appears after the if statement.

• Logical operators. To combine conditions we can use the three logical operators AND, OR, and NOT represented by &&, ||, and !, respectively.

  • The AND operator only returns true if both sides of the expression are true.

  • The OR operator returns true when either one or both sides of an expression are true.

  • The NOT operator reverses the logic of an expression.

• The switch statement. This compares the result of an expression with a series of possible cases, similar in effect to a multiple if statement.

• Looping with for, for...in, while, and do...while. It's often necessary to repeat a block of code a number of times, something JavaScript enables by looping.

  • The for loop. Useful for looping through code a certain number of times, the for loop consists of three parts: the initialization, test condition, and increment parts. Looping continues while the test condition is true. Each loop executes the block of code and then executes the increment part of the for loop before re-evaluating the test condition to see if the results of incrementing have changed it.

  • The for...in loop. This is useful when we want to loop through an array without knowing the number of elements there are in the array. JavaScript works this out for us so that no elements are missed.

  • The while loop. This is useful for looping through some code so long as a test condition remains true. It consists of a test condition and the block of code that's executed only if the condition is true. If the condition is never true, then the code never executes.

  • The do...while loop. This is similar to a while loop, except that it executes the code once and then keeps executing the code as long as the test condition remains true.

  • break and continue statements. Sometimes we have a good reason why we want to break out of a loop prematurely, in which case we need to use the break statement. On hitting a break statement, code execution stops for the block of code marked out by the curly braces and starts immediately after the closing brace. The continue statement is similar to break, except that while code execution stops at that point in the loop, the loop is not broken out of, but instead continues as if the end of that reiteration had been reached.

• Functions are reusable bits of code. JavaScript has a lot of built-in functions, which provide us programmers a service, such as converting a string to a number. However, JavaScript also allows us to define and use our own functions using the function keyword. Functions can have zero or more parameters passed to them and can return a value if we so wish.

• Variable scope and lifetime. Variables declared outside of a function are available globally, that is, anywhere in the page. Any variables defined inside a function are private to that function and can't be accessed outside of it. Variables have a lifetime, the length of which depends on where the variable was declared. If it's a global variable, its lifetime is that of the page—while the page is loaded in the browser, the variable remains alive. For variables defined in a function, the lifetime is limited to the execution of that function. Once the function has finished being executed, the variables die, and their values are lost. If the function is called again later in the code, the variables will be empty.