const

const.js

// cannot re-assign a const variable
const x = "written in stone";
x = "changed my mind"; // raises TypeError: Assignment to constant variable

// but you can still mutate a const variable
const person = {};
person.name = "Steve";

/*
`const` is great for assigning to a module that is required into code, 
 so as to prevent the variable holding the module from being accidentally reassigned:
*/

// note 'require' is provided by node.js
const important_library = require("important_library");
important_library = "I totally forgot this was const";