JavaScript判断对象、数组是否包含某个属性、某个值的方法

const obj = { name: 'John', age: 25 };
console.log('name' in obj); // 输出: true
console.log('gender' in obj); // 输出: false

const obj = { name: 'John', age: 25 };
console.log(obj.hasOwnProperty('name')); // 输出: true
console.log(obj.hasOwnProperty('gender')); // 输出: false

const obj = { name: 'John', age: 25 };
console.log(obj.name !== undefined); // 输出: true
console.log(obj.gender !== undefined); // 输出: false

const obj = { name: 'John', age: 25 };
console.log(Object.keys(obj).includes('name')); // 输出: true
console.log(Object.keys(obj).includes('gender')); // 输出: false

 let obj = {
     name: '再努力些吧',
     age: 18,
     work: '前端',
 }
 // 旧写法
 console.log('age' in obj);//true
 console.log('sex' in obj);//false
 // 新写法
 console.log(Reflect.has(obj, 'age'));//true
 console.log(Reflect.has(obj, 'sex'));//false

/* 如下例子我就不写了引用别人的。作者：Netmad,来源：知乎,
链接：https://www.zhihu.com/question/21907133/answer/378501127  */
function foo() {
  this.id = 'id';
}
foo.prototype.common = 'common';
var o = new foo();
'id' in o; // true
'common' in o; // true
'whatever' in o; // false
o.hasOwnProperty('id'); //true
o.hasOwnProperty('common'); //false
o.propertyIsEnumerable('id'); //true
o.propertyIsEnumerable('common'); //false
// 目前为止, hasOwnPerproty和propertyIsEnumerable看上去没啥差别
// 通过Object.defineProperty定义新的属性
Object.defineProperty(o, 'prop', {
  value: 'valueOfProp',
  enumerable: false
});
o.prop; // valueOfProp
o.hasOwnProperty('prop'); // true
o.propertyIsEnumerable('prop'); //false
// 如果defineProperty时enumerable为true, 那么这里依然和hasOwnProperty一样

const arr = [1, 2, 3, 4, 5];
console.log(arr.includes(3)); // 输出: true
console.log(arr.includes(6)); // 输出: false

const arr = [1, 2, 3, 4, 5];
console.log(arr.indexOf(3) !== -1); // 输出: true
console.log(arr.indexOf(6) !== -1); // 输出: false

const arr = [1, 2, 3, 4, 5];
console.log(arr.find(element => element === 3) !== undefined); // 输出: true
console.log(arr.find(element => element === 6) !== undefined); // 输出: false

const arr = [1, 2, 3, 4, 5];
console.log(arr.some(element => element === 3)); // 输出: true
console.log(arr.some(element => element === 6)); // 输出: false

const arr = [1, 2, 3, 4, 5];
console.log(arr.findIndex(element => element === 3)); // 输出: 2
console.log(arr.findIndex(element => element === 6)); // 输出: -1