JSFiddle - React, Tailwind, and code Playground

by Gonzalo

JavaScript

/*
Declaring Variables and Functions

Variables can be defined with the var statement, and are initialised to undefined:
*/
var a; // undefined
var b = 'hello';
/*
The scoping of these statements is dependent on where they’re declared. If var statements don’t appear inside a function, they’re globally accessible:
*/
var a = 1;

function sum(b) {
  return a + b;
}
/*
Missing var

Problems start to occur when a var statement is forgotten:
*/
function example() {
  a = 1;
  b = 1;
  return a + b;
}
/*
These variables are not local to the example function, they’re actually global. If a or b already existed, then their values will be overwritten.

Accidentally leaving out a var statement is surprisingly easy, and could potentially cause irritating bugs.
*/
/*
Variable Declaration Styles

Some people like to group var statements together:
*/
var a = 1,
    b = 2,
    c = 3;
/*
What would happen if a comma was missed by mistake?
*/
function example() {
  var a = 1
      b = 2,
      c = 3;
}

example();

console.log(typeof a);
console.log(typeof b);
console.log(typeof c);
/*
Running this will show that while a is undefined and therefore local to example, the other variables are global.

The reason some developers place commas before lists of variable declarations is to make it easier to see if a comma has been forgotten:
*/
var a = 1
  , b = 2
  , c = 3
  ;