Turns out the solution using interfaces only works for static types, not generic ones. (see what I did there?). It also makes the compiler ignore when you forget to specify a type for each function parameter, so I don't recommend it. I still hope you enjoy reading my article and get some inspiration for hacking around with TypeScript. The one thing you can do is limiting the type checking hole to this function, by forcing the caller to cast the output. So our type Flatten will look something like this: type Flatten
= NonObjectPropertiesOf & SubPropertiesOf; 1. Functions can also include parameter types and return type. Typescript does have some polyfills, depending on the target and lib you are using. If it makes you feel any better, we can give it a fancy name like “finite recursion”. With this option, you can create new keys and filter keys based on the inputs: We want to create a mapped type that accepts an arbitrary nested JSON-like type such as this one: The goal is to preserve the primitives and Arrays, but flatten the objects such that the properties are now on root level: You might be wondering why my colleague wanted to do this. A flatten function that accepts nested number arrays and returns a flat number array. Here is a list of the features of an array − 1. What’s going on here? As another example, we could also write a type called Flatten that flattens array types to their element types, but leaves them alone otherwise: type Flatten < T > = T extends any[] ? Let’s first get all the values of our object, then filter them down to the ones of type object while again making the exception for Arrays. We will then look at how we can use conditional types to create a single multi-purpose Flatten type, that can dynamically chose a flattening strategy based on the type … And we can abbreviate some of our repeating variables so they fit on a single line , So there it is: the least ugly DeepFlatten I can think of. Spoiler alert: the other half is not going to be as easy. T [number] : T ; // Extracts out the element type. Inferring Within Conditional Types My guess is that there is no typing for these methods, and I did try to npm run -dev @types/array.prototype.flatmap but still not solve. A recursive deep flatten would in theory be infinite: it would keep flattening until there is nothing left to flatten. Less cheating: create a file called types.d.ts at the root of your source directory containing declare module "*";. 5. Flatten javascript objects into a single-depth object - Object Flatten. Ok, so mapping over ObjectValuesOf doesn’t really give us what we want. Do you have a more elegant solution? Or… is it? Well, it turns that keyof T gives us the “union of the known, public property names of T”. Even page 2 of Google results showed no hope of a good solution — so the only logical conclusion to draw is that this must be madness. It assumes that everything on your destination type is meant to be mapped. Do we really have types that has object nested more than 4 levels deep? Object-Oriented language: TypeScript provides a complete feature of an object-oriented programming language such as classes, interfaces, inheritance, modules, etc. We now get a union of all objects on our input type. type Flatten = NonObjectPropertiesOf & SubPropertiesOf; type NonObjectPropertiesOf = Pick>; type UnionToIntersection = (U extends any, type DeepFlatten = Pick> &, union of the known, public property names of T, Best Practices for Structuring Express Apps, Verifying JSON Web Tokens with Express-JWT, How to implement local fulfillment for Google Assistant actions using Dialogflow, Why you should stop installing npm packages globally, Track Your Smartphone in 2D With JavaScript, TypeScript Generics — Recover your type checks and IntelliSense. ^3.0.0. When Flatten is given an array type, it uses an indexed access with number to fetch out string[]’s element type.Otherwise, it just returns the type it was given. A quick search for “typescript deep flatten type” showed no obvious answers. Please be aware that this article was written for older versions of TypeScript. We’ll also use the distributive conditional types (extends any ?) One might be able to use the same constructs to do other kinds of flattening. Among guitar players, there’s a joke everyone should understand. Do we really have types that has object nested more than 4 levels deep? again to make sure our intermediate types are properly distributed: Yeah I know… not the prettiest of types. Without recursive types, we‘d have to write a ‘finite’ version of this calling FlattenUnion -> FlattenUnion2 -> FlattenUnion3 for each nested object in a union) By doing this, we now have a type that allows us to safely access any property on either side of a union. In the case of the union of our baz and wobble objects, it will only give us the keys that are known to be on both these objects. If so, how about 10 levels? A recursive deep flatten would in theory be infinite: it would keep flattening until there is nothing left to flatten. It is a language designed for large-scale JavaScript application development, which can be executed on any browser, any Host, and any Operating System. Let’s try to map over ObjectValuesOf to get all sub-properties: Let’s check the type of SubPropertiesOf: So this gives us an empty object type. It assumes that everything on your destination type is meant to be mapped. But do we really need that? An array declaration allocates sequential memory blocks. type Str = Flatten ; // ^ = type Str = string // Leaves the type alone. Why? Since TypeScript can't know what all the possible types are going to be, it cannot type check that without evaluating the code. Step one in learning TypeScript: The basics types. When appropriate and possible, a corresponding flag will be added to disable that behavior. Before we dive into deep flattening a type, let’s simplify the problem by creating a shallow flatten type first. I wanted to do const { name, age } = body.value I tried adding the string and number types like this: const { name: string, age: number } = body.value But this didn’t work. For example, I recommend checking out Recursive Conditional Types in the TypeScript changelog. It's not a problem with VS Code per se, but the fact that VS Code by default uses bundled TS version to power JS/TS completion. (Note: this is using recursive types, a new feature of Typescript 3.7. So for now, it doesn’t seem possible to write a DeepFlatten type that references itself. In order to also extract the deeply nested properties, we will also need to pass our child objects through Flatten recursively. Let’s first get all the values of our object, then filter them down to the ones of type object while again making the exception for Arrays. Arrays are static. But do we really need that? I was using TypeScript in Deno to build a sample project and I had to destructure an object. Jest does not run webpack, so if you do not import corejs 2. A quick search for recursive types may point you to a comment on the TypeScript Github with a possible solution: reference back using an interface. In TypeScript, we can write code for both client-side as well as server-side development.
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