typeof null javascript

In this article, we have discussed JavaScript typeof in detail. The following example shows the details: For the example shown above, here are the results: To see if an object is an array or contains the word, we can use the constructor property. The typeof operator in JavaScript evaluates and returns a string with the data type of an operand. A fix was proposed for ECMAScript (via an opt-in), but was rejected. The type tags were stored in the lower bits of the units. The old Internet Explorer was the only browser known to implement additional return values, before the spec removed the behavior of typeof returning implementation-defined strings for non-callable non-standard exotic objects. // Bienvenido al editor en linea de TypeScript!, este es un sitio web. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? The typeof operator returns " object " for arrays because in JavaScript arrays are objects. // Number tries to parse things into numbers, // including values that cannot be type coerced to a number, // note that a number within a string is still typeof string, // String converts anything into a string, safer than toString, // Boolean() will convert values based on if they're truthy or falsy, // two calls of the ! If null is a primitive, why does typeof(null) return "object"? For example, to find the type of 123, we would write -, This will return a string with the type of 123, which, in this case, will be "number". The type tag for objects was 0. null was represented as the NULL pointer ( 0x00 in most platforms). Summary. The typeof operator has higher precedence than binary operators like addition (+). You can consider it a bug in JavaScript that typeof null is an object. // Puedes pensarlo de tres maneras: //. Douglas Crockford calls it a mistake. It is supposed to be something that doesn't exist. Values were kept in 32 bit units in this version, which included a short type tag (1-3 bits) and the actual data of the value. One thing we should remember is that empty values are not related to undefined because they have a value and a type. typeof is an operator that categorizes primitives and helps distinguish them from objects The "typeof null" problem is a holdover from JavaScript's original version. Why do quantum objects slow down when volume increases? Reasons 2: Insert script tag before declaring DOM elements. This list of values is exhaustive. The ECMAScript specification identifies these language data types: 6.1.1 The Undefined Type But since the implementation of JavaScript predates the writing of the ECMAScript spec, and the specification was careful not to correct foibles of the initial implementation, there's still a legitimate question about why it was done this way in the first place. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). How do I test for an empty JavaScript object? These methods are also known as mutator methods. Answer (1 of 2): Linked list are special list of some data elements linked to one another .the logical ordering is represented by having each elements pointing to the next element. typeoftypeof 6.1.2 The Null Type Properties such as Array.prototype.length result in counting the number of elements in the array. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. The typeof operator returns a string indicating the type of the operand's value. See the following example. We should note that the typeof in js is not a variable so it does not have a data type. Area Bermain membuat Anda bisa menulis TypeScript atau JavaScript secara daring dengan aman dan bisa dibagikan. This would typically mean that the type of null should also be "null". Summing up, we should keep note of the following: Evidently, JavaScript typeof is useful when you are looking to return the type of a variable. Why is the federal judiciary of the United States divided into circuits? Connect and share knowledge within a single location that is structured and easy to search. in <x {.y} /> , y is an object spread) because JSX doesn't provide syntax for directly passing a . typeof is generally always guaranteed to return a string for any operand it is supplied with. However, the result of typeof operators in JavaScript will always return a string. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. It doesn't make intuitive sense so why would they use it? And nowhere in the spec does it say typeof should return anything but "undefined", "object", "boolean", "number", "string", "function", and "symbol" (ECMAScript 2015). Once discovered, the decision was made to enshrine the bug rather than fix it as changing the output could break existing code that depended on the erroneous behavior. Examples of frauds discovered because someone tried to mimic a random sequence. From the MDN page about the behaviour of the typeof operator: null // This stands since the beginning of JavaScript typeof null === 'object'; In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. each element is called node which has two parts. Lets see the following example of an empty string: In the example above, the value is not undefined and the typeof JavaScript is the string. No spec-compliant engines are reported to produce (or had historically produced) values other than those listed. The null value is technically a primitive, the way "object" or "number" are primitives. The example shown below explains these conditions: The results for the example shown above are mentioned below: As mentioned above, the typeof operator in JavaScript will return with an object for JavaScript typeof array because arrays are objects. For all the JavaScript variables, the constructor property brings us a constructor function. The reason for this is, in computing, NaN is actually technically a numeric data type. JavaScript typeof [1,2,3,4] object null JavaScript null "" null typeof null object null : var person = null ; // null (), undefined : var person = undefined; // undefined, undefined undefined Aapt2 Error: Check Logs for Details (Reasoning and Solutions), Initializer Element Is Not Constant: Way To Error Elimination, Actioncontroller::invalidauthenticitytoken: A Way To Premium Solutions, Failed To Set up Listener: SocketException: Address Already in Use, OSError: [Errno 48] Address Already in Use: Four Solutions, Link JavaScript to HTML: The Most Straightforward Method to Use, JavaScript Switch Guide: Learn How To Use Switch Seamlessly. operator, SyntaxError: redeclaration of formal parameter "x". For objects, arrays, and JavaScript typeof null, it returns an object. Frequently asked questions about MDN Plus, MDN Web Docs , typeof JavaScript , : ECMAScript 2019 typeof , Internet Explorer , JavaScript JavaScript 0 null NULL ( 0x00) null 0 typeof "object" (), ECMAScript () typeof null === 'null' , ECMAScript 2015 typeof typeof 'undefined' typeof , let In fact, the ECMAScript specification defines null as the primitive value that represents the intentional absence of any object value (ECMA-262, 11.4.11). document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Position Is Everything provides the readers with Coding and Computing Tips & Tutorials, and Technology News. An expression representing the object or primitive whose type is to be returned. (reference). In JavaScript, arrays are technically objects; just with special behaviours and abilities. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null, https://2ality.com/2013/10/typeof-null.html. IE 678 , IE (tc39/ecma262#1440 (comment)), Last modified: 2022103, by MDN contributors. // The name of the constructor; for example `Array`, `GeneratorFunction`, // `Number`, `String`, `Boolean` or `MyCustomClass`. It should be null. The case of document.all having type "undefined" is classified in the web standards as a "willful violation" of the original ECMAScript standard for web compatibility. Are the S&P 500 and Dow Jones Industrial Average securities? Use //# instead, TypeError: can't assign to property "x" on "y": not an object, TypeError: can't convert BigInt to number, TypeError: can't define property "x": "obj" is not extensible, TypeError: can't delete non-configurable array element, TypeError: can't redefine non-configurable property "x", TypeError: cannot use 'in' operator to search for 'x' in 'y', TypeError: invalid 'instanceof' operand 'x', TypeError: invalid Array.prototype.sort argument, TypeError: invalid assignment to const "x", TypeError: property "x" is non-configurable and can't be deleted, TypeError: Reduce of empty array with no initial value, TypeError: setting getter-only property "x", TypeError: X.prototype.y called on incompatible type, Warning: -file- is being assigned a //# sourceMappingURL, but already has one, Warning: 08/09 is not a legal ECMA-262 octal constant, Warning: Date.prototype.toLocaleFormat is deprecated, Warning: expression closures are deprecated, Warning: String.x is deprecated; use String.prototype.x instead, Warning: unreachable code after return statement, Interaction with undeclared and uninitialized variables, Custom method that gets a more specific type. Why typeof null is 'object' in javascript? It's used in Object.prototype.toString(). In the above examples, it's pretty straightforward what the type of the operands will be. Consequently, null had 0 as type tag, hence the typeof return value "object". <null_variable> === null is the best way to strictly check for null. Lets see this with the help of an example: From the example shown above, the results would be as follows: The following should be noted when using typeof in JavaScript: Now, lets move on to the primitive data in typeof JavaScript. These are: If we talk about object data types, we have: Lastly, we have these two data types unable to contain any values. operator, SyntaxError: redeclaration of formal parameter "x". The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). typeof null was never intended to yield anything other than "null". Therefore, their types are 'object'. Why does Cauchy's equation for refractive index contain only even power terms? This is why the typeof a Class, is still just a Function. Ever since Microsoft created its own JavaScript engine and copied all the features and bugs of the first engine version, all subsequent engines copied this bug and now it's too late to fix it. // Parentheses can be used for determining the data type of expressions. Take heart! These objects have different patterns and abilities. As the word goes, null means nothing. In simple terms, primitive data value includes a single value without any additional methods or properties. It would have resulted in typeof null === 'null'. The following example shows such a variable: Just like the previous example shown above, this value will be undefined as well and so would be the type. 2. // String toString , // ! Frequently asked questions about MDN Plus. The typeof JavaScript operator works with function and object types. We know that the "typeof" null in javascript is an object, and we also know that it's a bug in JavaScript. For more information about types and primitives, see also the JavaScript data structure page. 3. console.log(typeof null) // object. Before Classes, to make an inheritable object, we would have to use a function. The types of operand needed are also found in the string. Note, typeof is an operator, not a function (and in fact you can omit the parentheses around what comes after it), so it does not makes sense to talk about passing by reference here. Now lets see the data types in the typeof operator in JavaScript. So, the name "Not a Number", doesn't mean that the value is not numeric. For greater specificity in checking types, here we present a custom type(value) function, which mostly mimics the behavior of typeof, but for non-primitives (i.e. // This stands since the beginning of JavaScript. Zorn's lemma: old friend or historical relic? However in JavaScript, it is considered an object, and we can set it to null. This is a bug remnant from javascript's first version. Avoid them. That means JavaScript counts null as an object. For objects, arrays, and JavaScript typeof null, it returns an object. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Lets start with an undefined value example. don't trust everything is written in a book. All articles are written by Ire Aderinokun, frontend developer and user interface designer. Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? I wanted to know what kind of bug it is, and why anyone hasn't resolved it yet. null javascriptjavascript null nulltypeof"object" JavaScript JavaScript . For example -. BCD tables only load in the browser with JavaScript enabled. ( reference) This also explains why not all NaN values are equal. In short, both null and undefined have equal values but different types. doesn't make sense why people would use this as a null check anyway. 6.1.5 The Symbol Type To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The production UnaryExpression : typeof UnaryExpression is evaluated as follows: As has been pointed out, the spec says so. ( reference) Use our guide and its examples diligently, and youll surely master the use of typeof. The book "JavaScript: The Good Parts" actually mentions the fact that typeof null === 'object' in section A.6 of appendix A entitled 'Awful Parts'. To understand the JavaScript typeof array, lets see the following function isArray example: Now that we have discussed the functions and the constructor properties, lets move to the undefined, empty values, and null values. What is the type of a typeof expression? In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. Receive quality articles written by Ire Aderinokun, frontend developer and user interface designer. Those two NaN values are not equal because they are not necessarily the same unrepresentable number. A fix was proposed for ECMAScript (via an opt-in), but was rejected. Consequently, null had 0 as type tag, hence the "object" typeof return value. in <x y /> , y is a string) or an object spread (e.g. The following table summarizes the possible return values of typeof. I'm guessing you're actually looking for empty strings, in which case this simpler code will work: if (!pass || !cpass || !email || !cemail || !user) { Which will check for empty strings ( "" ), null, undefined, false and the numbers 0 and NaN. Although the null is a data type in JavaScript, typeof null returns "object". The type tag for objects was 0. null was represented as the NULL pointer ( 0x00 in most platforms). Use //# instead, TypeError: can't assign to property "x" on "y": not an object, TypeError: can't convert BigInt to number, TypeError: can't define property "x": "obj" is not extensible, TypeError: can't delete non-configurable array element, TypeError: can't redefine non-configurable property "x", TypeError: cannot use 'in' operator to search for 'x' in 'y', TypeError: invalid 'instanceof' operand 'x', TypeError: invalid Array.prototype.sort argument, TypeError: invalid assignment to const "x", TypeError: property "x" is non-configurable and can't be deleted, TypeError: Reduce of empty array with no initial value, TypeError: setting getter-only property "x", TypeError: X.prototype.y called on incompatible type, Warning: -file- is being assigned a //# sourceMappingURL, but already has one, Warning: 08/09 is not a legal ECMA-262 octal constant, Warning: Date.prototype.toLocaleFormat is deprecated, Warning: expression closures are deprecated, Warning: String.x is deprecated; use String.prototype.x instead, Warning: unreachable code after return statement. I'd love to know what it is between (boolean, string, number, array, object, function, symbol, null, undefined, NaN), I recommend doing a find and replace on your code base going from, It's a shame this change didn't at least make it into strict mode, People have been taking advantage of the quirk, and many code out there will have to be changed if this was not rejected, I guess. In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. Lets see the difference between the complex data types in the following section. null === undefined evaluates as false because they are not, in fact, equal. However, it is a numeric data type whose value cannot be represented using actual numbers. In JavaScript, typeof null is 'object', which incorrectly suggests that null is an object. The TypeError: Cannot read property 'getAttribute' of Null in JavaScript. JavaScript typeof is a keyword used to return the type of a variable. In APIs, null is often retrieved in a place where an object can be expected but no object is relevant. Finally, we have Classes. In this version, values were stored in 32 bit units, which consisted of a small type tag (1-3 bits) and the actual data of the value. Answer: Because the spec says so. SyntaxError: Unexpected '#' used outside of class body, SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**', SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. As a result of this similarity, null has the 0 type tag, which corresponds to an object. Not the answer you're looking for? Therefore, parentheses are needed to evaluate the type of an addition result. For instance, [1,2] instanceof Array will evaluate to true. 6.1.6 Numeric Types The type of an undeclared variable is undefined However, it does not return objects for JavaScript typeof function. //. typeof. This can be seen by logging in as 1 user, creating a TODO item, then logging out and logging back in as another user - you will see the TODO item created by the first user in the second user's list. The typeof JavaScript operator works with function and object types. Classes were introduced in ES2015 (ES6) as a better syntax for prototype-based inheritance. Visit Mozilla Corporations not-for-profit parent, the Mozilla Foundation.Portions of this content are 19982022 by individual mozilla.org contributors. In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. Detecting if object is null in Javascript - why doesn't this work? Since starting, JavaScript values were represented as the type tag and a value. Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? Due to an early bug, however, it yielded "object" instead. (logical NOT) operator are equivalent to Boolean(), // use Array.isArray or Object.prototype.toString.call, // to differentiate regular objects from arrays. Arrays also have special methods, e.g. null == undefined evaluates as true because they are loosely equal. const let const typeof typeof ReferenceError , document.all undefined , document.all 'undefined' ECMA JavaScript , typeof typeof([]) typeof(new Date()) typeof(/abc/) 'object' , typeof obj , ReferenceError typeof nonExistentVar === 'undefined' . Even with undeclared identifiers, typeof will return "undefined" instead of throwing an error. Lets now see the effect of null and undefined below: As we can see from the example used above, the first null would bring us false and the second null would return true. In addition to "number", the typeof operator can return one of 6 potential results -. From the MDN page about the behaviour of the typeof operator: In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. Lets get started. Thus, there is no value. Lets see how to do that in the following example: In the example shown above, we have made the person null, taking away all the information mentioned earlier. SyntaxError: test for equality (==) mistyped as assignment (=)? This is generally regarded as a mistake. The typeof operator accepts an operand and returns the type of the provided operand as a string. Ready to optimize your JavaScript with Rust? In JavaScript, typeof null is an object which gives a wrong impression that, null is an object where it is a primitive value. 6.1.7 The Object Type. Hence, using the typeof operator is not recommended to JavaScript compare to null. 2022 Position Is Everything All right reserved, Primitive Data Types Using Typeof in JavaScript, Typeof in JavaScript: Complex Data Values, JavaScript Typeof: Undefined, Empty, Null Values, Emptying an Object by Making it Undefined. typeof null === "object"; In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. All articles written with by Ire Aderinokun, JavaScript Array Methods - Mutator Methods. // The following are confusing, dangerous, and wasteful. Just like null, we can also empty an object by making it undefined. ?` unparenthesized within `||` and `&&` expressions, SyntaxError: for-in loop head declarations may not have initializers, SyntaxError: function statement requires a name, SyntaxError: identifier starts immediately after numeric literal, SyntaxError: invalid assignment left-hand side, SyntaxError: invalid regular expression flag "x", SyntaxError: missing ) after argument list, SyntaxError: missing ] after element list, SyntaxError: missing } after function body, SyntaxError: missing } after property list, SyntaxError: missing = in const declaration, SyntaxError: missing name after . typeof null; JavaScript Check if Null Example: typeof null Analysis As discussed earlier, we do not use typeof operator as it determines the type of a value that evaluates to the object. The same function is actually being created, but just with the author writing it in a different, cleaner way. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Array.prototype.push() or Array.prototype.unshift() (See JavaScript Array Methods - Mutator Methods). The best method to go for is the strict equality operator. Lets quickly see an example of such an array: The first line would return true and the second line would bring us false. To know how a function returns variables, we must take a look at the constructor property. Although document.all is also falsy and loosely equal to undefined, it is not undefined. Checking if a key exists in a JavaScript object? When working with variables, we should note that they can be emptied if the value is set to undefined. For historical reasons the typeof operator is not consistent with this categorisation in two cases: Another operator -- instanceof -- can be used to know whether an object inherits from a certain prototype. I really love that book, however, I cannot consider that a bug because the ECMA specification for JavaScript state that the type of null must be an object. The Data Type of typeof The typeof operator is not a variable. In other words, null is often used to signify an empty reference to an object. This would typically mean that the type of null should also be "null". Typeof null is object. This means that, regardless of resultant type, the type of a typeof [any operand], will always be a string. If the resultant type of the expression is, for example, a number, what will be returned is "number". This is a long-standing bug in JS, but one that is likely never going // object's class. Content available under a Creative Commons license. The value null is written with a literal: null . Lets see how that look like: This time, both the value and the type would be undefined. These properties are always either strings (e.g. JSX elements are translated to JavaScript function calls and JSX element attributes are translated to properties on a JavaScript object literal. The application seems to work, in that items are added/removed to/from the list, but in fact it is treating content with a "null" _currentPrincipalId. All constructor functions called with new will return non-primitives ("object" or "function"). However, Classes have always just been a syntactical wrapper around the function method. null is not an identifier for a property of the global object, like undefined can be. Object.is(<null_variable>,null) is an equally reliable way to check for null. Reasons 1: Using method getAttribute () on a DOM element which doesn't exist. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Consequently, null had 0 as type tag, hence the typeof return value "object". // Symbol.toStringTag often specifies the "display name" of the. Now, lets move to empty values and see the difference. Now the change can't be added because of bad coding. The data is a reference to an object . typeof null. fixing it would cause a lot more bugs! Most return objects, with the notable exception being Function, which returns a function. How do I correctly clone a JavaScript object? Operators ( + - * / ) do not have any data type. I'm reading 'Professional Javascript for Web Developers' Chapter 4 and it tells me that the five types of primitives are: undefined, null, boolean, number and string. 6.1.4 The String Type The change proposed for typeof null will break existing code. typeof null // "object" The null value is technically a primitive, the way "object" or "number" are primitives. Kiro Risk thinks it kinda sorta makes sense: The reasoning behind this is that null, in contrast with undefined, was (and still is) often used where objects appear. However, it does not return objects for JavaScript typeof function. Articles on frontend development and more. Where does the idea of selling dragon parts come from? All current browsers expose a non-standard host object document.all with type undefined. Find centralized, trusted content and collaborate around the technologies you use most. For example, arrays have a Array.prototype.length property, which will return the number of elements in the array. However, this is not the case because of a peculiarity with the way JavaScript was first defined. INFO part which stored the information and POINTER which stores t. Unfortunately, in JavaScript, the data type of null is an object. SyntaxError: Unexpected '#' used outside of class body, SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**', SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. The JavaScript typeof array is an object just like the arrays in JavaScript as technical objects. The "typeof null" bug is a remnant from the first version of JavaScript. In the first implementation of JavaScript, values were represented in two parts - a type tag and the actual value. Wouldn't that mean that null is passed by reference (I'm assuming here all objects are passed by reference), hence making it NOT a primitive? However, there are a few cases where it is unclear or misunderstood what the type of the operand should be. JavaScript (javascript) In JavaScript, null is a primitive value of the null type. Lets see that in the example shown below: The first line would bring us an undefined value and the second line (typeof null) would result in an object. Why is the eastern United States green if the wind moves from west to east? Is JavaScript a pass-by-reference or pass-by-value language? What is the most efficient way to deep clone an object in JavaScript? 6.1.3 The Boolean Type 6.1.6.1 The Number Type are deprecated, SyntaxError: "use strict" not allowed in function with non-simple parameters, SyntaxError: "x" is a reserved identifier, SyntaxError: a declaration in the head of a for-of loop can't have an initializer, SyntaxError: applying the 'delete' operator to an unqualified name is deprecated, SyntaxError: cannot use `? Enable JavaScript to view data. Why typeof (null) == 'object' in JavaScript? As mentioned earlier, undefined values belong to a variable without any value. The following sample code shows the typeof operator in use: const str = 'Im a string'; console.log(typeof str); // Expected output: string. // que te brinda la oportunidad de escribir, compartir y aprender TypeScript. Is it possible to hide or delete the new Toolbar in 13.1? So what should we be using instead of typeof to check for the type of value a variable is holding? However, even without a value, the type is still an object. Block scoped variables are in a temporal dead zone from the start of the block until the initialization is processed, during which it will throw an error if accessed. We have five types of data that contain values in JavaScript. Consequently, null had 0 as type tag, hence the typeof return value "object". to be fixed. Moreover, its a useful operator that aids in checking the type of variable used in a code. Instead, null expresses a lack of identification, indicating that a variable points to no object. In the example above, const str is a String and serves as the operand for the typeof operator. To my knowledge, you can call methods on anything other than, @peter you cannot call methods on a string primitive, but thankfully string primitives (and number primitives and boolean primitives) are implicitly and automatically "auto-boxed" in String, Number, and Boolean wrappers when you use one of the primitives with a property reference operator (, Since I can't find now the video I'll post this just for curious people and without any reference: Crockford explained how a zero value on resolving type of null pointed to the zero indexed element on the types array, so this was a clear developing bug which the Microsoft guys accidentally propagated when decompiling and recompiling JS for their browser. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? Save my name, email, and website in this browser for the next time I comment. One way to determine whether a value is an object, is to use the Object function: I think it is too late to fix typeof. let userName = null; console.log (typeof (userName)); // object. But unfortunately, this returns "object" because of an historical bug that cannot be fixed. typeof is very useful, but it's not as versatile as might be required. However, its type is not null but object because JavaScript treats null as an empty object: type = typeof null ; console .log (type); Code language: JavaScript (javascript) Technically, JavaScript treats functions as objects. There are four types of typeof operator in JavaScript that can result in a primitive data value. // At this point there's no robust way to get the type of value, Enumerability and ownership of properties, Error: Permission denied to access property "x", RangeError: argument is not a valid code point, RangeError: repeat count must be less than infinity, RangeError: repeat count must be non-negative, RangeError: x can't be converted to BigInt because it isn't an integer, ReferenceError: assignment to undeclared variable "x", ReferenceError: can't access lexical declaration 'X' before initialization, ReferenceError: deprecated caller or arguments usage, ReferenceError: reference to undefined property "x", SyntaxError: "0"-prefixed octal literals and octal escape seq. objects and functions), it returns a more granular type name where possible. 17. // app.js console.log(typeof null === 'object'); The output is true. However, using typeof on lexical declarations (let const, and class) in the same block before the line of declaration will throw a ReferenceError. ( NOT) 2 Boolean() , // Array.isArray Object.prototype.toString.call , // , // obj toPrototypeString() , // ([object HTMLDivElement] ), // (Android 2.3 )