JavaScript Variables

    What are Variables in JavaScript

    When we write the JavaScript code we deal with many types of data it could be a number, name, date, speed value, etc., and to use these values we require variables. A variable is a data container or storage which holds the data value for us. To create a variable in JavaScript we write the variable name followed by the assignment operator = and the variable value. Example

    name = 'Rahul';
    or 
    var name = 'Rahul';
    or
    let name ='Rahul';

    All three statements can be used to create or declare a variable in JavaScript. In the above JavaScript code name is the variable, = symbol is assignment operator and 'Rahul' is the data value. As JavaScript is a loosely typed language, you can or cannot use semicolon ; at the end of statements. The let and var are the JavaScript keywords which we will be going to discuss in another tutorial.

    What are Constants in JavaScript?

    Constants are the special variables, a normal variable can be declared with or without keywords like var and let and their value can be changed throughout the program. But a constant variable is defined with const keyword and its value remains the same throughout the program and if you try to change it, the JavaScript engine or interpreter will throw an error. Example

    const name2 = "Ravi";    //constant variable 
    
    name2= "Raghav";   // error

    JavaScript Variable Naming Rules

    We can not give any name to the variables, there are some set of rules we need to follow when we give a name to our variable. Rule 1: A Variable name can only start with alphabets, $ (dollar) or _ (underscore) symbol.

    Name1 = "Rahul";      //valid
    name1 = "Rahul";      //valid
    &name = "Rahul";      // valid
    _name = "Rahul";      // valid
     
    1name = "Rahul";      // invalid
    &name = "Rahul";      // Invalid
    

    Rule 2: A reserved JavaScript Keyword can not be used as a variable name. There are 63 reserved keywords in JavaScript.

    let let = "Rahul";  // invalid
    var let = "Rahul";  // invalid

    Rule 3: A variable name must be a sequence of alphabets, numbers, underscore, and $ dollar symbol.

    name$_12 = "Rahul";    //valid 
    name&_12 = "Rahul";    // invalid

    Rule 4: There must not be any space or gap in the variable name.

    name 1 ="Rahul";     //invalid
    name1 = "Rahul";     // valid

    What is Variable Declaration in JavaScript?

    When we define a variable in our JavaScript code without assigning any value, that is called Variable Declaration in JavaScript. To declare a variable in JavaScript we can use special keywords such as var and let .

    let name;   // variable declaration
    var let;    // variable declaration

    When we only declare a variable without assigning any value, JavaScript treats that variable as undefined .

    What is Variable Initialization in JavaScript?

    When we assign an initial value to a variable, that is known as variable Initialization in JavaScript. The variable initialization can be done with or after the variable declaration. Example

    const name = "Rahul";       // initialization with declaration
    
    let age;                 // declaration
    age = 20;                // initialization
    

    Keywords in JavaScript

    There are 64 words reserved in JavaScript which we can not use for variable names. These 64 reserved words are also known as JavaScript keywords, we will be discussing all of these keywords in the further JavaScript tutorial article, for now, here is the list of all 64 JavaScript keywords.

    abstract arguments await boolean
    break byte case catch
    char class const continue
    debugger default delete do
    double else enum eval
    export extends false final
    finally float for function
    goto if implements import
    in instanceof int interface
    let long native new
    null package private protected
    public return short static
    super switch synchronized this
    throw throws transient true
    try typeof var void
    volatile while with yield

    Summary

    • Variable is a name that is used as a container for the data value.
    • A JavaScript variable can be declared and initialize using either of the three keywords const, let, and var.
    • We need to follow the Naming variables rules while giving a name to the variable.
    • A keyword is a reserved word that has a special functionality and meaning in the JavaScript program.
    • There are 64 keywords present in JavaScript.
    • JavaScript is a loosely typed programming language, so we can choose between putting a semicolon; at the end of every statement.

    People are also reading: