typeof.undefine.js

// Good practice to verify if a var exist or not or is the value is the one expected

var x = 0;

if ( typeof x !== 'undefined') {
  console.log('Exist');
} else {
  console.log('Not exist');
}

// Redux
function counter(state, action) {
  if (typeof state === 'undefine') {
    return 0;
  }
  if (action.type === 'INCREMENT') {
    return state + 1
  } else if(action.type === 'DECREMENT') {
      return state - 1;
  } else {
    return state;
  }
}

//Redux - ES6

const counter = (state = 0, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMET':
      return state - 1;
    default:
      return state;
  }
}