The const keyword was introduced in ES6 (2015) . Variables defined with const cannot be Redeclared. Variables defined with const cannot be Reassigned. Variables defined with const have Block Scope. Cannot be Reassigned A const variable cannot be reassigned : Example const PI = 3.141592653589793 ; PI = 3.14 ; // This will give an error PI = PI + 10 ; // This will also give an error Must be Assigned JavaScript const variables must be assigned a value when they are declared: Correct const PI = 3.14159265359 ; Incorrect const PI; PI = 3.1 4159 265359 ; When to use JavaScript const? As a general rule, always declare a variable with const unless you know that the value will change. Use const when you declare: A new Array A new Object A new Function A new RegExp Constant Objects and Arrays The keyword const is a little misleading. It does not define a constant value. It defines a constant reference to a value.
4 Ways to Declare a JavaScript Variable: Using var Using let Using const Using nothing What are Variables? Variables are containers for storing data (storing data values). In this example, x , y , and z , are variables, declared with the var keyword: Example var x = 5 ; var y = 6 ; var z = x + y; In this example, x , y , and z , are variables, declared with the let keyword: Example let x = 5 ; let y = 6 ; let z = x + y; In this example, x , y , and z , are undeclared variables: Example x = 5 ; y = 6 ; z = x + y; From all the examples above, you can guess: x stores the value 5 y stores the value 6 z stores the value 11 When to Use JavaScript var? Always declare JavaScript variables with var , let , or const . The var keyword is used in all JavaScript code from 1995 to 2015. The let and const keywords were added to JavaScript in 2015. If you want your code to run in older browser, you must use var . When to Use JavaScript const?