Is undefined an object in javascript?
ex: string is an object because string.length exists.
4 Answers
undefined is a primitive:
The global undefined property represents the primitive value undefined. It is one of JavaScript's primitive types.
Strings are primitives too. When you use <referenceToString>.<someProperty>, the interpreter wraps the string in an object wrapper so that it can reference String.prototype.
answered Dec 2, 2019 at 9:09
CertainPerformance
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Comments
No, undefined is a primitive, the only instance of the Undefined type. Details in the specification, but the primitive types are (currently):
- Undefined
- Null
- Boolean
- Number
- BigInt
- String
- Symbol
(It's likely more will be added, such as BigDecimal.)
Everything else is an object.
ex: string is an object because string.length exists.
Strings are not usually objects, they're primitives that are coerced to their object equivalent when necessary (in theory; in practice, JavaScript engines optimize away that conversion). For instance:
let s = "foo";
s is a primitive string, typeof s is "string". But when you try to access a property on it (s.length for instance, or s.toUpperCase()), the primitive string is coerced to a String object. (Again: in theory — and sometimes in practice, particularly in loose mode.) That's how it gets access to its methods, which are defined on String.prototype. You can also create a String object intentionally: new String("foo") but there's almost never any good reason to do that.
This same is true for all of the other primitive types except Undefined and Null, which have no object type equivalent (if you try to access a property on undefined or null, you get a TypeError). (This is a bit confusing for Null, because typeof null is "object" and null is used when something object-typed needs a "no value" value. But Null is a primitive type, and null is a primitive value.)
4 Comments
Felix Kling
I missed that
BigInt is already done... have to pay more attention :D
T.J. Crowder
@FelixKling - :-) My money is on BigDecimal in ES2021 as well.
T.J. Crowder
@adiga - Thanks. You'd be truly shocked how often I make that typo when not typing in an IDE... :-)
Suraj Jain
why does
typeof null gives object, and why does Object.prototype.toString.call(undefined) gives [object Undefined]4.3.2 primitive value
member of one of the types Undefined, Null, Boolean, Number, Symbol, or String
6.1 ECMAScript Language Types
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.
3 Comments
T.J. Crowder
Thanks for contributing! But this just repeats what previous answers have already stated, which isn't useful. It's also out of date (missing BigInt). The up-to-date, living specification is here. It's best to link to that unless you're talking about a specific version for some reason. But again, this just repeats what previous answers have said.
T.J. Crowder
The draft is started immediately after the last snapshot is forked, then proposals that reach Stage 4 are added to it as part of reaching Stage 4. According to caniuse, BigInt is implemented in recent versions of V8 (Chrome, Chromium, Brave, etc., and the Chromium version of Edge currently in beta), SpiderMonkey (Firefox), and in the JavaScriptCore in the latest Safari Technical Preview.
undefined is a property of the global object; i.e., it is a variable in global scope. The initial value of undefined is the primitive value undefined. A variable that has not been assigned a value is of type undefined. Eg. In the following code, the variable x is not initialized, and the if statement evaluates to true.
var x;
if (x === undefined) {
// these statements execute
}
else {
// these statements do not execute
}
3 Comments
Shubham Vernekar
undefined is a property of the global object; i.e., it is a variable in global scope. The initial value of undefined is the primitive value undefined.
Kevin.a
Then you should edit your answer and say 'global object' instead of the object , to make it clearer
T.J. Crowder
@ShubhamVernekar "undefined is a property of the global object" Yes, that's true, but it's not really an answer to the question. "i.e., it is a variable in global scope" You want to be a bit careful there, not all globals are properties of the global object (not since ES2015). :-)
typeof(undefined), and you'll get "undefined". String is also a function, and not an object.string.lengthis not evidence thatstringis an object: javascriptweblog.wordpress.com/2010/09/27/…stringmight be. We can't see how it is defined..lengthargument.