Javascript Variables

Variables are declared with the var keyword. JavaScript is dynamically typed so every variable can hold a value of any data type.

Variables can be declared without an initial value.

Some example declarations:

var foo;
var bar = 42;
var foo, bar, baz;
var foo = 42,
    bar = 'baz',
    z;

Variables that don't explicitly get assigned an initial value have the value undefined.

const foo = 42;
foo = 21; // error or ignored

const foo; // error
foo = 42;

Variable names

Valid characters for variable names include a wide range of unicode characters. However, the name must start with a letter, _ or $. Not doing so will result in a syntax error.

Examples:

var π = 3.141;
var _foo = π;
var 0_bar = '...'; // Syntax error

Variable access

Trying to read an undeclared variable results in a runtime error:

var foo;
console.log(bar); // ReferenceError: bar is not defined.

However, writing to an undeclared variable is valid by default. It will create an implicit global variable and should thus be avoided:

function foo() {
    bar = 42;
}
foo();
console.log(bar); // no error

'use strict';

JavaScript has 6 data types. Five of those are so called primitive data types:

• Boolean
• Number
• String
• Null
• Undefined

Everything else that is not a value of one of the above types is an

• Object

As we will see in the following slides, objects belong to different kinds of "classes" of objects.