typeof運算符用于確定變量或表達式的數據類型，返回字符串如"number"、"string"、"object"等；其返回值包括"undefined"、"boolean"、"number"、"string"、"bigint"、"symbol"、"function"和"object"；1. typeof null返回"object"是歷史遺留的bug；2. 要安全檢查null應使用value === null；3. typeof適用于檢查變量是否定義、驗證函數參數、調試等場景；4. 對數組和函數typeof返回"object"和"function"，但無法準確區(qū)分數組與對象，需用Array.isArray()；5. 使用typeof時需注意其局限性和邊緣情況。

The typeof operator in JavaScript is used to determine the data type of a variable or expression. It returns a string indicating the type, such as "number", "string", or "object". While it seems straightforward, there are nuances that can trip people up if they're not aware.

What values does typeof return?

There are only a handful of possible return values from typeof. Here’s what you might see:

• "undefined" — for undeclared variables or variables that haven’t been assigned.
• "boolean" — for true or false.
• "number" — for any numeric value, including NaN.
• "string" — for string values.
• "bigint" — for large integers created with the BigInt type.
• "symbol" — for values created using Symbol().
• "function" — for functions (though technically functions are objects).
• "object" — for objects, arrays, and null.

Wait—yes, typeof null returns "object". That's one of those quirks we’ll cover next.

Why does typeof null return "object"?

This is a classic gotcha. If you run:

typeof null // returns "object"

You might expect "null" as the result, but this behavior has been around since the early days of JavaScript. It’s basically a historical bug that’s now part of the language due to backward compatibility. So keep in mind: typeof null gives "object" — don’t rely on typeof alone to check for null.

To safely check for null, use:

if (value === null) { ... }

That way, you avoid confusion between objects and actual null values.

When is typeof useful?

Despite its quirks, typeof comes in handy in several scenarios:

• Checking if a variable is defined:

  if (typeof myVar === 'undefined') { ... }

• Validating function parameters:
  You can make sure a value passed into a function is of the expected type before doing operations on it.

• Debugging:
  During development, typeof helps confirm whether a value is what you expect — especially when dealing with dynamic inputs or third-party data.

However, if you need more precise type checking (like distinguishing arrays from objects), consider using Array.isArray() or instanceof.

What about functions and arrays?

When you use typeof on a function, it returns "function":

typeof function() {} // "function"

That’s actually helpful because functions are technically objects in JavaScript, but typeof treats them as a special case.

For arrays:

typeof [1, 2, 3] // "object"

So again, typeof isn't enough to distinguish arrays from regular objects. Use Array.isArray() for that.

Well, that’s how typeof works — simple, occasionally surprising, but definitely useful once you know its limits. Just remember to double-check edge cases like null and arrays.基本上就這些。

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