JavaScript syntax is the set of rules, how JavaScript programs are constructed:// How to create variables:
var x;
let y;
// How to use variables:
x = 5;
y = 6;
let z = x + y;
JavaScript Values
The JavaScript syntax defines two types of values:
- Fixed values
- Variable values
Fixed values are called Literals.
Variable values are called Variables.
JavaScript Literals
The two most important syntax rules for fixed values are:
1. Numbers are written with or without decimals:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Numbers</h2>
<p>Number can be written with or without decimals.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = 10.50;
</script>
</body>
</html>
Result:
JavaScript Numbers
Number can be written with or without decimals.
10.5
2. Strings are text, written within double or single quotes:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Strings</h2>
<p>Strings can be written with double or single quotes.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = 'John Doe';
</script>
</body>
</html>
Result:
JavaScript Strings
Strings can be written with double or single quotes.
John Doe
JavaScript Variables
In a programming language, variables are used to store data values.
JavaScript uses the keywords var, let and const to declare variables.
An equal sign is used to assign values to variables.
In this example, x is defined as a variable. Then, x is assigned (given) the value 6:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Variables</h2>
<p>In this example, x is defined as a variable.
Then, x is assigned the value of 6:</p>
<p id="demo"></p>
<script>
let x;
x = 6;
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript Variables
In this example, x is defined as a variable. Then, x is assigned the value of 6:
6
JavaScript Operators
JavaScript uses arithmetic operators ( + - * / ) to compute values:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Operators</h2>
<p>JavaScript uses arithmetic operators to compute values (just like algebra).</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = (5 + 6) * 10;
</script>
</body>
</html>
Result:
JavaScript Operators
JavaScript uses arithmetic operators to compute values (just like algebra).
110
JavaScript uses an assignment operator ( = ) to assign values to variables:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>Assigning JavaScript Values</h2>
<p>In JavaScript the = operator is used to assign values to variables.</p>
<p id="demo"></p>
<script>
let x, y;
x = 5;
y = 6;
document.getElementById("demo").innerHTML = x + y;
</script>
</body>
</html>
Result:
Assigning JavaScript Values
In JavaScript the = operator is used to assign values to variables.
11
JavaScript Expressions
An expression is a combination of values, variables, and operators, which computes to a value.
The computation is called an evaluation.
For example, 5 * 10 evaluates to 50:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Expressions</h2>
<p>Expressions compute to values.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = 5 * 10;
</script>
</body>
</html>
Result:
JavaScript Expressions
Expressions compute to values.
50
Expressions can also contain variable values:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Expressions</h2>
<p>Expressions compute to values.</p>
<p id="demo"></p>
<script>
var x;
x = 5;
document.getElementById("demo").innerHTML = x * 10;
</script>
</body>
</html>
Result:
JavaScript Expressions
Expressions compute to values.
50
The values can be of various types, such as numbers and strings.
For example, “John” + ” ” + “Doe”, evaluates to “John Doe”:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Expressions</h2>
<p>Expressions compute to values.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = "John" + " " + "Doe";
</script>
</body>
</html>
Result:
JavaScript Expressions
Expressions compute to values.
John Doe
JavaScript Keywords
JavaScript keywords are used to identify actions to be performed.
The let keyword tells the browser to create variables:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>The <b>let</b> Keyword Creates Variables</h2>
<p id="demo"></p>
<script>
let x, y;
x = 5 + 6;
y = x * 10;
document.getElementById("demo").innerHTML = y;
</script>
</body>
</html>
Result:
The let Keyword Creates Variables
110
The var keyword also tells the browser to create variables:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>The var Keyword Creates Variables</h2>
<p id="demo"></p>
<script>
var x, y;
x = 5 + 6;
y = x * 10;
document.getElementById("demo").innerHTML = y;
</script>
</body>
</html>
Result:
The var Keyword Creates Variables
110
In these examples, using var or let will produce the same result.
You will learn more about var and let later in this tutorial.
JavaScript Comments
Not all JavaScript statements are “executed”.
Code after double slashes // or between /* and */ is treated as a comment.
Comments are ignored, and will not be executed:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Comments are NOT Executed</h2>
<p id="demo"></p>
<script>
let x;
x = 5;
// x = 6; I will not be executed
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript Comments are NOT Executed
5
You will learn more about comments in a later chapter.
JavaScript Identifiers / Names
Identifiers are JavaScript names.
Identifiers are used to name variables and keywords, and functions.
The rules for legal names are the same in most programming languages.
A JavaScript name must begin with:
- A letter (A-Z or a-z)
- A dollar sign ($)
- Or an underscore (_)
Subsequent characters may be letters, digits, underscores, or dollar signs.
Note
Numbers are not allowed as the first character in names.
This way JavaScript can easily distinguish identifiers from numbers.
JavaScript is Case Sensitive
All JavaScript identifiers are case sensitive.
The variables lastName and lastname, are two different variables:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript is Case Sensitive</h2>
<p>Try to change lastName to lastname.</p>
<p id="demo"></p>
<script>
let lastname, lastName;
lastName = "Doe";
lastname = "Peterson";
document.getElementById("demo").innerHTML = lastName;
</script>
</body>
</html>
Result:
JavaScript is Case Sensitive
Try to change lastName to lastname.
Doe
JavaScript does not interpret LET or Let as the keyword let.
JavaScript and Camel Case
Historically, programmers have used different ways of joining multiple words into one variable name:
Hyphens:
first-name, last-name, master-card, inter-city.
Hyphens are not allowed in JavaScript. They are reserved for subtractions.
Underscore:
first_name, last_name, master_card, inter_city.
Upper Camel Case (Pascal Case):
FirstName, LastName, MasterCard, InterCity.
Lower Camel Case:
JavaScript programmers tend to use camel case that starts with a lowercase letter:
firstName, lastName, masterCard, interCity.
JavaScript Character Set
JavaScript uses the Unicode character set.
Unicode covers (almost) all the characters, punctuations, and symbols in the world.