JavaScript is a sequence of statements to be executed by the browser. Unlike HTML, JavaScript is case-sensitive; therefore, watch your capitalization closely when you write JavaScript statements and create or call variables, objects, and functions.
JavaScript Statements
A JavaScript statement is a command to a browser. The purpose of the command is to tell the browser what to do.
The following JavaScript statement tells the browser to write “Hello Dolly” to the Web page:
document.write("Hello Dolly");
It is normal to add a semicolon at the end of each executable statement. Most people think this is a good programming practice, and most often you will see this in JavaScript examples on the Web.
The semicolon is optional (according to the JavaScript standard), and the browser is supposed to interpret the end of the line as the end of the statement.
Using semicolons makes it possible to write multiple statements on one line, although good programming practice encourages placing only one statement per line.
JavaScript Code
JavaScript code (or just JavaScript) is a sequence of JavaScript statements. Each statement is executed by the browser in the sequence it is written.
<html><body><script type="text/javascript">document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");</script></body></html>
JavaScript Blocks
JavaScript statements can be grouped together in blocks. Blocks start with a left curly bracket { and end with a right curly bracket }. The purpose of a block is to make the sequence of statements execute together.
<html><body><script type="text/javascript">{document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");}</script></body></html>
The preceding example is not very useful. It just demonstrates the use of a block. Normally, a block is used to group statements together in a function or in a condition (in which a group of statements should be executed if a condition is met).
JavaScript Comments
JavaScript comments can be added to explain the JavaScript script or to make the code more readable. Single line comments start with //. The following example uses single-line comments to explain the code.
<html><body><script type="text/javascript">// Write a headingdocument.write("<h1>This is a heading</h1>");// Write two paragraphs:document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");</script></body></html>
JavaScript Multiline Comments
Multiline comments start with /* and end with */. The following example uses a multiline comment to explain the code.
<html><body><script type="text/javascript">/*The code below will writeone heading and two paragraphs*/document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");</script></body></html>
Using Comments to Prevent Execution
In the following example, the comment is used to prevent the execution of a single code line (can be suitable for debugging):
<html><body><script type="text/javascript">//document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");</script></body></html>
In the following example, the comment is used to prevent the execution of a code block (can be suitable for debugging):
<html><body><script type="text/javascript">/*document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");*/</script></body></html>
Using Comments at the End of a Line
In the following example, the comment is placed at the end of a code line.
<html><body><script type="text/javascript">document.write("Hello"); // Write "Hello"document.write(" Dolly!"); // Write " Dolly!"</script></body></html>
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