Variables are containers for storing data values. In JavaScript, you can declare variables using var, let, and const.

• var (Older way, generally avoid in modern JS):

  • Function-scoped or globally-scoped.
  • Can be re-declared and updated.
  • Variables declared with var are hoisted (their declaration is moved to the top of their scope).

  var name = "Alice";
  var name = "Bob"; // Re-declaration allowed
  name = "Charlie"; // Update allowed
  

• let (Modern way, preferred for variables that might change):

  • Block-scoped (scope is limited to the block {...} where they are defined).
  • Cannot be re-declared in the same scope.
  • Can be updated.
  • Not hoisted before initialization (Temporal Dead Zone).

  let age = 30;
  // let age = 31; // Error: Cannot re-declare block-scoped variable 'age'.
  age = 31; // Update allowed
  

• const (Modern way, preferred for variables that should not change):

  • Block-scoped.
  • Cannot be re-declared or re-assigned after initialization.
  • Must be initialized during declaration.
  • Good for values that are constant, like mathematical constants or configuration settings.

  const PI = 3.14159;
  // PI = 3.14; // Error: Assignment to constant variable.
  // const PI = 3.14; // Error
  

Recommendation: Use const by default. If you know a variable needs to be reassigned, use let. Avoid using var in modern JavaScript development due to its sometimes confusing scoping and hoisting behavior.