Deep clone in TypeScript (preserving types)
Solution 1
Typescript isn't discarding type information here. In the DefinitelyTyped lodash.d.ts file, you can see that cloneDeep
is defined as
cloneDeep<T>(
val: T,
customizer?: (value: any) => any,
thisArg?: any
) : T
Ignoring the arguments we don't care about, it takes a T
as input, and spits out a T
as output. So Typescript isn't losing any type information; it considers the output of cloneDeep
to be the same type as the input.
You should be able to verify this via your editor: assuming you have some editor that lets you inspect the type of variables or autocompletes methods (which I'd highly recommend, if you don't).
Why then is the typeof
not working as you expect? It's because the Typescript type information doesn't carry over to runtime. instanceof
is a native JS operator, which Typescript doesn't change the behavior of, which you can see by running this snippet:
"use strict";
class A {}
let a = new A();
let b = _.cloneDeep(a);
if (b instanceof A) {
alert("b is an instance of A");
} else {
alert("b is not an instance of A");
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.js"></script>
The reason that b instanceof A
is false is that instanceof
is checking against constructors: x instanceof A
returns true if the function A
is a constructor somewhere in x's prototype chain (see the MDN documentation on instanceof). Lodash, however, doesn't use constructors when it clones objects. It can't. (How would it know what arguments to pass?) It creates a plain JS object that has all the methods of the cloned object, but doesn't reproduce it's prototype chain.
Lodash's clone
(and most of lodash's methods, really) is best used when dealing with raw JS Objects. If you're using it in conjunction with constructors and instanceof
checking things get a bit murky.
One solution here is to avoid the instanceof
checking, and do something akin to duck typing; don't check that the object's constructor is a particular function, but check that the object has the properties that you expect it to.
Another solution is, as suggested in the comments, implement a clone method on your class itself, which wouldn't use lodash.
class A() {
clone() {
var cloned = new A(); //pass appropriate constructor args
//make other necessary changes to make the state match
return cloned;
}
}
Solution 2
There is an interesting blog post about deep cloning http://blog.soulserv.net/understanding-object-cloning-in-javascript-part-ii/. You can use the author's implementation of the deep cloning clone()
function:
class SomeClass {
constructor(public test) {
}
}
let c = new SomeClass("Hey!");
c.test = "Hey!";
console.log(c instanceof SomeClass); // returns true
let cloneC = clone(c, /* resolve circular references */ true);
console.log(cloneC instanceof SomeClass); // returns true
[clone() source code] [Complete source code]
Solution 3
You can create your own deep clone using JSON stringify and parse methods:
export function deepClone<T>(obj: T): T {
return JSON.parse(JSON.stringify(obj)) as T;
}
Solution 4
You can use Lodash#cloneDeep utility. Example of use :
import * as _ from "lodash";
...
{
this.cloned = _.cloneDeep(data);
}
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Julian B
Updated on July 27, 2022Comments
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Julian B almost 2 years
I need to deep clone an object in TypeScript. This shouldn't be a problem as libraries like Lodash provide appropriate functions for that. However, these seem to discard type information.
> var a = new SomeClass(); > a instanceof SomeClass; < true > var b = _.cloneDeep(a); > b instanceof SomeClass; < false
Is there a way to clone objects in TypeScript while preserving this typing information?
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YOU over 8 yearssomething like
b: typeof a
? -
Ruan Mendes over 8 yearsAutomatically? No. You'd need to implement a clone method on the object and its nested objects.
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Bruno Grieder over 8 yearsPreserving Typescript types and preserving SomeClass.prototype is not the same thing. See @Retsam answer and developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…. Typescript types are erased at runtime.
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Retsam over 8 years@BrunoGrieder Good call on linking the instanceof MDN documentation; I've edited that into my answer.
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AsGoodAsItGets over 4 yearsI'm running the same test today (November 2019) with lodash 4.17.15 and it seems that the test is successful, i.e. instanceof returns true in both cases. Did something change in the meantime?
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Farshad Bayat almost 4 yearsbest solution and easy is here: stackoverflow.com/a/62969704/3913394
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MonsieurDart almost 5 yearsCaution with this: the date objects will be transformed into a string. I.e. its date type will not be kept.
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krillgar over 4 yearsThis answer, especially the section on TS types getting lost when executing in the browser, needs far more upvotes. Too many people don't understand that distinction.
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Arcsector about 3 yearsThe usage of
_.cloneDeep()
as opposed to just thecloneDeep()
function on it's own is what makes this answer great. -
TamusJRoyce almost 3 yearsnpmjs.com/package/lodash.clonedeep - there is also clonedeep stand-alone
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TamusJRoyce over 2 yearsThis is also not consistent in how it works across runtimes and browser environments