Confusion about typeof

Question

What is the typeof funtion?

Answer 1

The typeof operator returns a string indicating the type of the operand.

Answer 2

To check variable type(whether it is array, object, function etc.).

Answer 3

typeof is actually an operator, not a function, it returns you the type of the supplied operand, and returns you always^[1] a string value, that can be:

• "undefined"
• "object"
• "boolean"
• "number"
• "string"
• "function"

It can look like a function, because it can be applied to parenthesised expressions, but the parentheses are not mandatory, you can use it without them:

typeof expression

Is worth mentioning that you can reference even an identifier that is not declared (an unresolvable reference, e.g. a variable never declared) and it will not throw a ReferenceError, it will simply produce "undefined", for example:

if (typeof imUndeclared == 'undefined') {
  // Ok...
}

// Vs.

if (imUndeclared === undefined) { // ReferenceError!
}

A case that can cause confusion to newcomers to the language is that the typeof operator returns "object" for a null value, you should keep that in mind...

Also if you want to distinguish between Array objects, RegExp objects, Date objects, etc, it isn't too useful, but there are other ways to accomplish it.

---

^[1] With the infamous exception of JScript (IE) :), which can actually return "unknown"!!! for some host objects, e.g. typeof new ActiveXObject("Msxml2.XMLHTTP").abort; // "unknown"

Answer 4

typeof is a an unary operator (and not a function), defined by the ECMAScript standard and therefore implemented by most existing JavaScript implementations (Historically speaking, it was part of JavaScript since its version 1.1, before its ownership was handed over to ECMA International).

It is used to introspect the type of a variable by querying it (typeof expression) and checking the return value, which is a string object indicating the type with lossy precision (see below).

Usage

As per the ECMAScript 5 Standard (ECMA-262, p. 71), it will return the following string values when tested with these given parameters of the following types:

• Undefined --> "undefined"
• Null --> "object"
• Boolean --> "boolean"
• Number --> "number"
• String --> "string"
• Object --> it depends:
  • native and does not implement Call --> "object"
  • native and does implement Call --> "function"
  • host and does not implement Call --> Implementation-defined except may not be "undefined", "boolean", "number", or "string"

For most implementations, typeof will also return:

• Array --> "object"
• Function --> "object" (see above Object's 2nd and 3rd case...)

(Note that types are capitalized, whereas their string identifier as returned by typeof are not)

Caveats

• As mentioned above, the type detection with typeof is lossy,
• typeof, as of ECMAScript 5, still doesn't support Array (which is identified as "object" in most cases. (see Douglas Crockford's Remedial JavaScript)
• Detection of Function and Array varies

Additional References

• Wikipedia's ECMAScript Syntax page will also provide usage examples of typeof.
• Mozilla's Developer Center's JavaScript Reference for typeof