JavaScript学习笔记(八) 数据类型

let a = 0
let b = 'hello'
let c = true
let d = null
let e = undefined
let f = {}
let g = []
let h = function(){}
function typeOf(value) {
    return Object.prototype.toString.call(value).slice(8, -1)
}
console.log('a: ' + typeOf(a))
console.log('b: ' + typeOf(b))
console.log('c: ' + typeOf(c))
console.log('d: ' + typeOf(d))
console.log('e: ' + typeOf(e))
console.log('f: ' + typeOf(f))
console.log('g: ' + typeOf(g))
console.log('h: ' + typeOf(h))
/*
 * 执行结果：
 * a: Number
 * b: String
 * c: Boolean
 * d: Null
 * e: Undefined
 * f: Object
 * g: Array
 * h: Function
**/

var Message = function(descrition) { // 构造函数
    this.detail = descrition
}
let message = new Message('Hello')
let object = new Object()
console.log(message instanceof Message)
console.log(message instanceof Object)
console.log(object instanceof Message)
console.log(object instanceof Object)
console.log(Message instanceof Object)
/*
 * 执行结果：
 * true
 * true
 * false
 * true
 * true
**/

function myInstanceOf(left, right) {
    let lValue = left.__proto__
    let rValue = right.prototype
    while (true) {
        if (lValue === null) return false
        if (lValue === rValue) return true
        lValue = lValue.__proto__
    }
}

let a = 1 + true
let b = 1 + null
let c = 1 + undefined
let d = '1' + true
let e = '1' + null
let f = '1' + undefined
let g = '1' + 1
let h = {} + true
let i = [] + false
console.log(a)
console.log(b)
console.log(c)
console.log(d)
console.log(e)
console.log(f)
console.log(g)
console.log(h)
console.log(i)
/*
 * 执行结果：
 * 2
 * 1
 * NaN
 * 1true
 * 1null
 * 1undefined
 * 11
 * [object Object]true
 * false
**/

// number -> string
let number = 123
let string = number + ''
console.log(typeof string)
console.log(string)
/*
 * 执行结果：
 * string
 * 123
**/

let number = 159
let string = 'hello'
let number_copy = number
let string_copy = string
number_copy = 258
string_copy = 'hi'
console.log('number: ' + number) // 原变量的值不会改变
console.log('string: ' + string) // 原变量的值不会改变
console.log('number_copy: ' + number_copy)
console.log('string_copy: ' + string_copy)
/*
 * 执行结果：
 * number: 159
 * string: hello
 * number_copy: 258
 * string_copy: hi
**/

对于引用类型：
赋值后新旧变量指向同一个内存地址，此时改变新变量会影响旧变量的值
let object = { first_name: 'Steve', last_name: 'Jobs' 

// 对于对象，常用的浅拷贝方式有：Object.assign
// 对于数组，常用的浅拷贝方式有：Array.from、slice、concat
let object = {
    name: {
        first_name: 'Steve',
        last_name: 'Jobs'
    },
    age: 18
}
let array = [[1, 3], 5, 7]
let object_shallow_copy = Object.assign({}, object)
let array_shallow_copy = Array.from(array)
object_shallow_copy['name']['first_name'] = 'Steven'
object_shallow_copy['age'] = 20
array_shallow_copy[0][0] = 2
array_shallow_copy[2] = 11
console.log('object["name"]["first_name"]: ' + object['name']['first_name'])
console.log('object["age"]: ' + object['age'])
console.log('array[0][0]: ' + array[0][0])
console.log('array[2]: ' + array[2])
console.log('object_shallow_copy["name"]["first_name"]: ' + object_shallow_copy['name']['first_name'])
console.log('object_shallow_copy["age"]: ' + object_shallow_copy['age'])
console.log('array_shallow_copy[0][0]: ' + array_shallow_copy[0][0])
console.log('array_shallow_copy[2]: ' + array_shallow_copy[2])
/*
 * 执行结果：
 * object["name"]["first_name"]: Steven
 * object["age"]: 18
 * array[0][0]: 2
 * array[2]: 7
 * object_shallow_copy["name"]["first_name"]: Steven
 * object_shallow_copy["age"]: 20
 * array_shallow_copy[0][0]: 2
 * array_shallow_copy[2]: 11
**/

function shallowCopy(object) {
    if (typeof object !== 'object') return
    let newObject = object instanceof Array ? [] : {}
    for (let key in object) {
        if (object.hasOwnProperty(key)) {
            newObject[key] = object[key]
        }
    }
    return newObject
}

// 最简单的方法就是 JSON.parse(JSON.stringify())，但是这种方法有一定的缺陷
// 如果属性中有 undefined、NaN、Infinity、function 类型，那么会转换为 null
// 如果属性中有 RegExp、Error 对象，那么会转换为空对象
let object = {
    name: {
        first_name: 'Steve',
        last_name: 'Jobs'
    },
    age: 18
}
let array = [[1, 3], 5, 7]
let object_deep_copy = JSON.parse(JSON.stringify(object))
let array_deep_copy = JSON.parse(JSON.stringify(array))
object_deep_copy['name']['first_name'] = 'Steven'
object_deep_copy['age'] = 20
array_deep_copy[0][0] = 2
array_deep_copy[2] = 11
console.log('object["name"]["first_name"]: ' + object['name']['first_name'])
console.log('object["age"]: ' + object['age'])
console.log('array[0][0]: ' + array[0][0])
console.log('array[2]: ' + array[2])
console.log('object_deep_copy["name"]["first_name"]: ' + object_deep_copy['name']['first_name'])
console.log('object_deep_copy["age"]: ' + object_deep_copy['age'])
console.log('array_deep_copy[0][0]: ' + array_deep_copy[0][0])
console.log('array_deep_copy[2]: ' + array_deep_copy[2])
/*
 * 执行结果：
 * object["name"]["first_name"]: Steve
 * object["age"]: 18
 * array[0][0]: 1
 * array[2]: 7
 * object_deep_copy["name"]["first_name"]: Steven
 * object_deep_copy["age"]: 20
 * array_deep_copy[0][0]: 2
 * array_deep_copy[2]: 11
**/