Variables, Constants & Comments

In JavaScript, variables are used to store values that can be used later in your code. Constants are similar to variables, but their values cannot be changed once they are set. Comments are used to add notes to your code that are ignored by the JavaScript engine.

• var is a keyword in JavaScript that was used to declare variables prior to the introduction of let and const. The var keyword has function scope, which means that a variable declared with var is accessible within the entire function in which it was declared, and can also be accessed within inner functions. One of the issues with var is that it is function scoped, but it also has a strange behavior called "hoisting". Hoisting refers to the behavior of moving variable and function declarations to the top of their respective scopes, which means that a variable declared with var can be used before it is declared in the code.

  console.log(x); // undefined
  var x = 10;

  Even though x is declared after the console.log statement, it still outputs undefined instead of throwing a ReferenceError. This is because the declaration of x is hoisted to the top of the function and the assignment of 10 remains in its original place. For these reasons, it's generally recommended to use let and const instead of var when declaring variables in JavaScript. let and const have block scope and do not exhibit the hoisting behavior of var

• Variables: Variables are containers for storing data values. In JavaScript, you can declare a variable using the var, let, or const keyword. The difference between var and let is that var has function scope while let has block scope. The const keyword is used to declare a constant that cannot be reassigned after it has been declared.

  let name = 'John Doe';
  const PI = 3.14;

• Constants: Constants are similar to variables, but their value cannot be changed once it has been assigned. In JavaScript, you can declare a constant using the const keyword.

  const PI = 3.14;

• Comments: Comments are used to add annotations and explanations to your code. They are ignored by the JavaScript interpreter and are only intended to be read by developers. In JavaScript, you can create single-line comments using // and multi-line comments using /* */.

  // This is a single-line comment
   
  /*
    This is a
    multi-line comment
  */