If you want to check whether a key is inside an Object or Array, you can use ‘in‘:
Object:
const obj = { name: ‘GraphQL‘, watched: false }; console.log(‘name‘ in obj); // true console.log(‘title‘ in obj); // false
Array:
const characters = [ ‘Harry Potter‘, ‘Ron Weasly‘, ‘Hermione Granger‘, ‘Nevel Longbottom‘, ‘Lavender Brown‘, ‘Scabbers‘, ‘Pigwidgeon‘, ] console.log(0 in characters); // true; console.log(6 in characters); // true;
[Javascript] Javascript 'in' opreator
原文：http://www.cnblogs.com/Answer1215/p/6275303.html