Let's talk about the difference between typeof and instanceof
Mar 11, 2022 am 11:25 AM
typeof和instanceof操作符都可用來判斷數(shù)據(jù)類型，那么它們之間有什么差異？下面本篇文章就來帶大家了解 typeof 和 instanceof ，聊聊它們的區(qū)別，希望對(duì)大家有所幫助！
typeof和instanceof操作符都是用來判斷數(shù)據(jù)類型的，但是它們的使用場(chǎng)景卻各不相同，其中一些細(xì)節(jié)也需要特別注意。接下來讓我們一探究竟，徹底掌握該知識(shí)點(diǎn)，再也不懼面試官的提問。
typeof
typeof是一個(gè)一元運(yùn)算符，放在一個(gè)運(yùn)算數(shù)前面，這個(gè)運(yùn)算數(shù)可以是任何類型。它返回一個(gè)字符串，說明運(yùn)算數(shù)的類型。請(qǐng)看栗子：
const type = typeof '中國(guó)萬歲'; // string
typeof 666; // number
typeof true; // boolean
typeof undefined; // undefined
typeof Symbol(); // symbol
typeof 1n; // bigint
typeof () => {}; // function
typeof []; // object
typeof {}; // object
typeof new String('xxx'); // object
typeof null; // object通過以上例子可以看出，typeof只能準(zhǔn)確判斷基本數(shù)據(jù)類型和函數(shù)（函數(shù)其實(shí)是對(duì)象，并不屬于另一種數(shù)據(jù)類型，但也能夠使用 typeof 進(jìn)行區(qū)分），無法精確判斷出引用數(shù)據(jù)類型（統(tǒng)統(tǒng)返回 object）。
有一點(diǎn)需要注意，調(diào)用typeof null返回的是object，這是因?yàn)樘厥庵?code>null被認(rèn)為是一個(gè)對(duì)空對(duì)象的引用（也叫空對(duì)象指針）。
如果想準(zhǔn)確判斷引用數(shù)據(jù)類型，可以用instanceof運(yùn)算符。
instanceof
instanceof運(yùn)算符放在一個(gè)運(yùn)算數(shù)的后面，給定對(duì)象的前面。它返回一個(gè)布爾值，說明運(yùn)算數(shù)是否是給定對(duì)象的實(shí)例：
const result = [] instanceof Array; // true
const Person = function() {};
const p = new Person();
p instanceof Person; // true
const message = new String('xxx');
message instanceof String; // true區(qū)別
typeof 會(huì)返回一個(gè)運(yùn)算數(shù)的基本類型，instanceof 返回的是布爾值
instanceof 可以準(zhǔn)確判斷引用數(shù)據(jù)類型，但是不能正確判斷基本數(shù)據(jù)類型
typeof 雖然可以判斷基本數(shù)據(jù)類型（null 除外），但是無法判斷引用數(shù)據(jù)類型（function 除外）
擴(kuò)展
Object.prototype.toString.call()
typeof和instanceof都有一定的弊端，并不能滿足所有場(chǎng)景的需求。如果需要通用檢測(cè)數(shù)據(jù)類型，可以使用Object.prototype.toString.call()方法：
Object.prototype.toString.call({}); // "[object Object]"
Object.prototype.toString.call([]); // "[object Array]"
Object.prototype.toString.call(666); // "[object Number]"
Object.prototype.toString.call('xxx'); // "[object String]"注意，該方法返回的是一個(gè)格式為"[object Object]"的字符串。
封裝函數(shù)
為了更方便的使用，我們可以將這個(gè)方法進(jìn)行封裝：
function getType(value) {
let type = typeof value;
if (type !== 'object') { // 如果是基本數(shù)據(jù)類型，直接返回
return type;
}
// 如果是引用數(shù)據(jù)類型，再進(jìn)一步判斷，正則返回結(jié)果
return Object.prototype.toString.call(value).replace(/^\[object (\S+)\]$/, '$1');
}
getType(123); // number
getType('xxx'); // string
getType(() => {}); // function
getType([]); // Array
getType({}); // Object
getType(null); // Null【相關(guān)推薦：javascript視頻教程、web前端】
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