typescript instanceof vs typeof

What is the difference between typeof and instanceof and when should one be used vs. the other?

stackoverflow.com › questions › 899574 › what-is-the-difference-between-typeof-and-instanceof-and-when-should-one-be-used

Use `instanceof` for custom types:

var ClassFirst = function () {};
var ClassSecond = function () {};
var instance = new ClassFirst();
typeof instance; // object
typeof instance == 'ClassFirst'; // false
instance instanceof Object; // true
instance instanceof ClassFirst; // true
instance instanceof ClassSecond; // false

Use `typeof` for simple built in types:

'example string' instanceof String; // false
typeof 'example string' == 'string'; // true

'example string' instanceof Object; // false
typeof 'example string' == 'object'; // false

true instanceof Boolean; // false
typeof true == 'boolean'; // true

99.99 instanceof Number; // false
typeof 99.99 == 'number'; // true

function() {} instanceof Function; // true
typeof function() {} == 'function'; // true

Use `instanceof` for complex built in types:

/regularexpression/ instanceof RegExp; // true
typeof /regularexpression/; // object

[] instanceof Array; // true
typeof []; //object

{} instanceof Object; // true
typeof {}; // object

And the last one is a little bit tricky:

typeof null; // object

Answer from Szymon Wygnański on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 899574 › what-is-the-difference-between-typeof-and-instanceof-and-when-should-one-be-used

What is the difference between typeof and instanceof and when should one be used vs. the other?

Use `instanceof` for custom types:

var ClassFirst = function () {};
var ClassSecond = function () {};
var instance = new ClassFirst();
typeof instance; // object
typeof instance == 'ClassFirst'; // false
instance instanceof Object; // true
instance instanceof ClassFirst; // true
instance instanceof ClassSecond; // false

Use `typeof` for simple built in types:

'example string' instanceof String; // false
typeof 'example string' == 'string'; // true

'example string' instanceof Object; // false
typeof 'example string' == 'object'; // false

true instanceof Boolean; // false
typeof true == 'boolean'; // true

99.99 instanceof Number; // false
typeof 99.99 == 'number'; // true

function() {} instanceof Function; // true
typeof function() {} == 'function'; // true

Use `instanceof` for complex built in types:

/regularexpression/ instanceof RegExp; // true
typeof /regularexpression/; // object

[] instanceof Array; // true
typeof []; //object

{} instanceof Object; // true
typeof {}; // object

And the last one is a little bit tricky:

typeof null; // object

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Both are similar in functionality because they both return type information, however I personally prefer instanceof because it's comparing actual types rather than strings. Type comparison is less prone to human error, and it's technically faster since it's comparing pointers in memory rather than doing whole string comparisons.

CoreUI

coreui.io › blog › what-is-the-difference-between-typeof-and-instanceof-in-javascript

What is the difference between typeof and instanceof in JavaScript · CoreUI

September 17, 2024 - Note: Both arrays and objects return 'object' when using typeof. To differentiate between them, you might need additional checks. The instanceof operator checks if an object is an instance of a specific class or constructor function.

Discussions

Difference Between typeof and instanceof in JavaScript | Interview Question

JS gives you 2 keywords to verify the type of the value referred to by a reference. First of all, you have the typeof keyword. This keyword, used in the form of typeof will return a string that takes values from the following list: number string undefined object undefined symbol bigint boolean As we can see, typeof returns a primitive type. An interesting corner case is typeof null === 'object' surprisingly. Every class instance will also return 'object' on a typeof call. If you want to test for a specific class type, you have to use the instanceof keyword. The syntax is instanceof . What the aforementioned expression will return is a boolean value - true/false, if the class is part of the prototype chain. const m = new Map(); console.log(m instanceof Map) // true console.log(m instanceof Set) // false class MyMap extends Map { } const mm = new MyMap(); console.log(mm instanceof Map) // true console.log(mm instanceof MyMap) // true console.log(mm instanceof Set) // false More on reddit.com

r/Angular2

September 30, 2022

typescript - Typeof/instanceof type alias - Stack Overflow

I wonder if it is possible to determine the type of an object in typescript. Please consider the example below: type T = [number, boolean]; class B { foo: T = [3, true]; bar(): boolean { return this.foo instanceof T; } } The typeof operator does not seem to be a solution, neither does instanceof. More on stackoverflow.com

stackoverflow.com

If typescript is duck-typed, how come when you use "instanceof", typescript accurately separate out the individual classes that implement the same interface? And how can we take advantage of this functionality in our typing?

instanceof tests *at runtime*, not at compile time, and it tests towards the *class* Error, and not the *interface* Error. Both exists and are defined in TS. This kind of situations is a bit of a mess, because a class is both a type at build time and a value at runtime (classes are just functions). More on reddit.com

r/typescript

October 23, 2024

type .kind checks vs. class instanceof checks

It looks like you're using the same example as the official documentation on Discriminated Unions , so I assume you've seen that section. Out of curiosity then, what makes you say this? However newer docs don't seem to ever show discriminated unions If you're looking to have discriminated unions like Rust, follow that section to roll your own or look for a library that's already implemented them. fp-ts has everything under the sun in a Haskell style, neverthrow has Rust-like Result and ResultAsync, and sum-types helps with the boilerplate of defining your own sum types. I wouldn't recommend instanceof checks, TypeScript just isn't designed to work well with nominal typing so it's not the best experience. More on reddit.com

r/typescript

May 8, 2022

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