在JavaScript中，使用typeof操作符可檢查變量的數(shù)據(jù)類型，但需注意其局限性。 1. typeof適用於基本類型：typeof 42返回"number"，typeof "hello"返回"字符串"，typeof true返回"boolean"，typeof undefined返回"undefined"，typeof Symbol()返回"symbol"，typeof function(){}返回"function"；2. typeof null返回"object"，這是歷史遺留問題，因此需用嚴格等於（===）單獨檢查null；3. 數(shù)組的typeof返回"object"，應(yīng)使用Array.isArray(arr)準確判斷是否為數(shù)組；4. 對於對象、日期或自定義構(gòu)造函數(shù)實例，可使用instanceof，如d instanceof Date返回true；5. 跨執(zhí)行環(huán)境最可靠的類型檢測方法是Object.prototype.toString.call(value).slice(8, -1)，如Object.prototype.toString.call([])返回"Array"，該方法能正確識別Null、Date、Array等類型。綜上，typeof適用於大多數(shù)基本類型檢查，但結(jié)合Array.isArray()、instanceof和Object.prototype.toString.call()可實現(xiàn)更精確的類型判斷。

In JavaScript, you can check the data type of a variable using the typeof operator. It's the most common and straightforward way to determine the type of a value or variable.

Use typeof for basic type checking

The typeof operator returns a string indicating the type of the operand. Here are the typical results:

 typeof 42 // "number"
typeof "hello" // "string"
typeof true // "boolean"
typeof undefined // "undefined"
typeof null // "object" (this is a known quirk!)
typeof Symbol() // "symbol"
typeof function(){} // "function"

Note: typeof null returns "object" due to a historical bug in JavaScript. So you need to be careful when checking for null .

Handle special cases like null and arrays

Since typeof has limitations, especially with null and objects, you may need additional checks:

 // Checking for null
if (variable === null) {
  console.log("It's null");
}

// Checking for arrays
Array.isArray(myArray) // true if it's an array

// typeof for arrays returns "object", which isn't specific
typeof [] // "object"
Array.isArray([]) // true

For objects and custom types, consider instanceof or Object.prototype.toString

If you're working with objects, dates, or custom constructors, instanceof can help:

 const d = new Date();
d instanceof Date // true

const arr = [1, 2, 3];
arr instanceof Array // true

For more reliable type detection (especially across different execution environments), you can use:

 Object.prototype.toString.call(value).slice(8, -1)
// Examples:
Object.prototype.toString.call([]) // "[object Array]" → "Array"
Object.prototype.toString.call(new Date) // "[object Date]" → "Date"
Object.prototype.toString.call(null) // "[object Null]" → "Null"

Summary of methods:

• ? typeof – best for primitives ( number , string , boolean , etc.)
• ?? Be cautious with typeof null → returns "object"
• ? Array.isArray() – reliable way to check arrays
• ? instanceof – good for custom objects and built-in constructors
• ? Object.prototype.toString.call() – most accurate for built-in types

Basically, typeof covers most day-to-day needs, but knowing its quirks helps you avoid bugs.

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