Variables in JavaScript are containers that store data values. Think of them as labeled boxes where each box has a name and can hold different types of information. For example, we might have a variable called name that stores the text Alice, an age variable storing the number 25, and an isStudent variable storing the boolean value true. Variables allow us to reference and manipulate data throughout our program using these symbolic names.
JavaScript provides three main keywords for declaring variables. The var keyword is the oldest method and creates function-scoped variables that can be redeclared and reassigned. The let keyword, introduced in ES6, creates block-scoped variables that can be reassigned but not redeclared in the same scope. The const keyword also creates block-scoped variables, but they cannot be reassigned after declaration and must be initialized when declared. Modern JavaScript development typically favors let and const over var for better scope management and code clarity.
Variables can be assigned values in several ways. You can declare and assign a value in one step, like var age equals 30, or let name equals Alice. Constants must be initialized when declared, such as const PI equals 3.14159. You can also declare a variable first and assign a value later, like declaring let city and then assigning it the value New York. Variables declared with var and let can be reassigned new values, but const variables cannot be changed after initialization.
JavaScript variable names must follow specific naming rules. They must start with a letter, underscore, or dollar sign, but cannot start with a number. Variable names are case-sensitive, meaning myVar and myvar are different variables. You cannot use reserved keywords like let, if, or function as variable names. Valid examples include userName, underscore private, dollar element, myAge2, and PI underscore VALUE. Invalid examples include 2name which starts with a number, user dash name which contains a hyphen, let which is a reserved keyword, my var which contains a space, and class which is also a reserved keyword. Following these rules ensures your code runs correctly and follows JavaScript conventions.
To summarize what we have learned about JavaScript variables: Variables are containers that store data values using symbolic names, making your code more readable and maintainable. Modern JavaScript development favors let and const over var for better scope management. Always follow proper naming conventions by starting with letters or underscores, avoiding spaces and reserved keywords. Remember that const variables must be initialized when declared and cannot be reassigned. Understanding variables is fundamental to JavaScript programming as they enable data manipulation and program logic throughout your applications.