JavaScript executes a program from start to finish in order in Java

Printing QR Code in Java JavaScript executes a program from start to finish in order

JavaScript executes a program from start to finish in order
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How to Do Everything with JavaScript
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FIGURE 2-2
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JavaScript will stop executing a program in order to execute the contents of a function
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But, for the most part, it is still fair to characterize a program as being executed in order from start to finish Figure 2-2 shows how the program flow jumps around depending on the code itself We can see that all the code inside a function is executed before the program continues where it left off
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Control Program Flow with Statements
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The HTML template we defined in 1 (see Create a JavaScript Template ) didn t really do anything useful, other than displaying some header text The only way we can get our JavaScript program to do anything is to add some statements A statement is the basic action item in any program code In effect, each statement is telling the computer to do something Statements can be divided up into five categories:
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Conditional Loops Object manipulation Comments Expressions
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The typical JavaScript program uses statements that fall into each of those categories Often, statements inside a program are organized into functions and classes as well, to make the program easier to manage and more efficient to develop We will examine functions in more detail later in this chapter, and classes in 5
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Execute Code Conditionally
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Computer programs almost always contain conditional statements There are two conditional statements in JavaScript: if and switch The if statement allows the program to choose one of two alternatives, based on some predefined factor In real life, you might decide that, if it were not raining, you would like to go to the baseball game Of course, if it were raining, you would then decide to stay home You can make that same decision in a JavaScript program with the following code:
CHAPTER 2: Learn JavaScript Fundamentals
function stay_or_go (raining) { if (raining == false) { return "Go to baseball game!"; } else { return "Got to stay home today"; } }
So you see, conditional statements give programs a choice between two or more alternatives in much the same way we make those choices in real life
The if Statement
Depending on how you use it, the if statement can be very simple or very complex The if statement can be used in the following ways
Syntax if (expression) {statements;} if (expression) {statements1;} else {statements2;} if (expression1) {statements1;} else if (expression2) {statements2;} else {statements3;} Use If expression evaluates to true, execute statements If expression evaluates to true, execute statements1 Otherwise, execute statements2 If expression1 evaluates to true, execute statements1 Otherwise, if expression2 evaluates to true, execute statements2 Otherwise, execute statements3
The if statement evaluates an expression to determine which set of statements to execute, if any Since the if statement expects a Boolean expression (one that evaluates to either true or false), it will try to convert expressions of other data types to either true or false JavaScript makes certain assumptions when converting from other data types to Boolean The strings true and false evaluate to the Booleans true and false The integers 1 and 0 are also converted to the Booleans true and false, respectively In computer programming terminology, an expression is a piece of code that, when evaluated, returns a value In JavaScript, the following can be used as expressions
Expression A variable A function that returns a value A literal Variables, functions, and literals combined using operators Example if ( x ) {statements;} if ( myfunc(x) ) {statements;} if ( true ) {statements;} if ( a > 5 ) {statements;}
How to Do Everything with JavaScript
Even the same if statement can be written in at least four different ways, all of which are valid:
if (expression) statement; if (expression)
statement;
if (expression) {statements;} if (expression) {
statements;
} The various preceding forms of the if statement are technically equivalent The first two can include only one statement Notice that the third and fourth forms use curly brackets, { and }, to enclose the statements Statements enclosed in curly brackets are generally treated as a group, called a statement block The if statement can include an optional else clause, to decide between one of two alternatives This form of the statement is sometimes called the if-else statement:
if (expression) {statements1;} else {statements2;}
The else clause allows you to specify a statement or group of statements that will be executed if the expression does not evaluate to true Only one else clause is allowed in any if statement Again, the statement can be used with or without the curly brackets Finally, the if statement can be used to choose between one of three or more alternatives With the introduction of the else-if clause, the if statement can include multiple expressions that will each be evaluated until either one of them evaluates to true or the else clause is encountered Multiple else-if clauses can be included, but keep in mind that the else-if clause must always precede any else clause
if (expression) { statements; } else if (expression) { statements; } else { statements; }
If you find yourself using more than two or three else-if clauses in a single if statement, you may want to consider using a switch statement instead, as described in the following section
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