Inference with conditional types

Conditional types are not just for switching behavior based on comparison — they can be used with an ‘infer’ keyword to access sub-parts of type information within a larger type

Type inference in conditional types

In the same release where conditional types were added to TypeScript a new infer keyword was added as well. This keyword, which can only be used in the context of a condition expression (within a conditional type declaration) is an important tool for being able to extract out pieces of type information from other types.

A motivating use case

Let’s consider a practical example: a class whose constructor wants a complicated options object, but doesn’t export the type for this object as an interface or type alias:

ts
class WebpackCompiler {
  constructor(options: {
    amd?: false | { [index: string]: any }
    bail?: boolean
    cache?:
      | boolean
      | {
          maxGenerations?: number
          type: "memory"
        }
    context?: string
    dependencies?: string[]
    devtool?: string | false
    entry?: string
    chunkLoading?: string | false
    dependOn?: string | string[]
    layer?: null | string
    runtime?: string
    wasmLoading?: string | false
    externalsType?:
      | "var"
      | "module"
      | "assign"
      | "this"
      | "window"
      | "self"
      | "global"
      | "commonjs"
      | "commonjs2"
      | "commonjs-module"
      | "amd"
      | "amd-require"
      | "umd"
      | "umd2"
      | "jsonp"
      | "system"
      | "promise"
      | "import"
      | "script"
 
    ignoreWarnings?: (
      | RegExp
      | {
          file?: RegExp
 
          message?: RegExp
 
          module?: RegExp
        }
    )[]
    loader?: { [index: string]: any }
    mode?: "development" | "production" | "none"
    name?: string
    parallelism?: number
    profile?: boolean
    recordsInputPath?: string | false
    recordsOutputPath?: string | false
    recordsPath?: string | false
    stats?:
      | boolean
      | "none"
      | "summary"
      | "errors-only"
      | "errors-warnings"
      | "minimal"
      | "normal"
      | "detailed"
      | "verbose"
    target?: string | false | string[]
    watch?: boolean
  }) {}
}Try

If, in our own code, we want to declare a variable to store our configuration before passing it into the compiler, we’re in trouble. Take a look below at how a spelling error around the word watch could create a tricky bug

ts
const cfg = {
  entry: "src/index.ts",
  wutch: true, // SPELLING ERROR!!
}
try {
  const compiler = new WebpackCompiler(cfg)
} catch (err) {
  throw new Error(
    `Problem compiling with config\n${JSON.stringify(
      cfg,
      null,
      "  "
    )}`
  )
}Try

What we really want here is the ability to extract that constructor argument out, so that we can obtain a type for it directly. Once we have that type, we’ll be able to change our code above to const cfg: WebpackCompilerOptions and we’ll have more complete validation of this object.

The infer keyword

The infer keyword gives us an important tool to solve this problem — it lets us extract and obtain type information from larger types.

Let’s simplify the problem we aim to solve, by working with a much simpler class:

ts
class FruitStand {
  constructor(fruitNames: string[]) {}
}Try

What we want is some kind of thing, where FruitStand is our input, and string[] is our result. Here’s how we can make that happen:

ts
type ConstructorArg<C> = C extends {
  new (arg: infer A, ...args: any[]): any
}
  ? A
  : neverTry

First, let’s establish that this works, and then we’ll unpack the syntax so that we can understand exactly what’s going on

ts
type ConstructorArg<C> = C extends {
  new (arg: infer A): any
}
  ? A
  : never
 
class FruitStand {
  constructor(fruitNames: string[]) {}
}
// our simple example
let fruits: ConstructorArg<typeof FruitStand>
      
let fruits: string[]
// our more realistic example
let compilerOptions: ConstructorArg<typeof WebpackCompiler>
          
let compilerOptions: {
    amd?: false | {
        [index: string]: any;
    };
    bail?: boolean;
    cache?: boolean | {
        maxGenerations?: number;
        type: "memory";
    };
    context?: string;
    ... 20 more ...;
    watch?: boolean;
}Try

Ok, this is great — let’s take a close look at how we did it by stepping through the syntax

First, we are creating a generic type, with a type param C which could be anything:

ts
type ConstructorArg<C> ...

Next, we’re beginning to define a conditional type, using the ternary operator syntax. We want to do something special if C looks like the static side of a class (the type with a constructor).

{ new (...args: any[]): any } is a type that matches any constructor signature, regardless of what arguments it may take, and what it instantiates:

ts
type ConstructorArg<C> = C extends {
  new (...args: any[]): any
}...

Next, we want to “collect” the first constructor argument. This is where the new infer keyword comes into play.

diff
- new (...args: any[]): any
+ new (arg: infer A, ...args: any[]): any

It kind of looks like we’re using a new type param (A) without including it next to <C> in the type parameter list. We also have an infer keyword to the left of A

ts
type ConstructorArg<C> = C extends {
  new (arg: infer A, ...args: any[]): any
}
  ? ...
  : ...

We should take note that our condition for this conditional type has changed. It will no longer match zero-argument constructors, but that’s fine because there’s nothing to extract in that case.

In the case where our condition matches type C, we’ll return the argument of type A that we “extracted” using that infer keyword.

ts
type ConstructorArg<C> = C extends {
  new (arg: infer A, ...args: any[]): any
}
  ? A
  : ...

And finally, in the case where type C is not a class we need to decide which type to “emit”. Ideally this will be something that, when used in a Union type (|), will kind of “disappear”.

ts
// for type `X` we're trying to figure out, we want...
;string | number | X // should just be `string | number`

What about any? Let’s see how that behaves

ts
let myValue: string | number | any
      
let myValue: anyTry

That’s not just the wrong result, it’s kind of the opposite result of what I was looking for. any, when used in a Union type, kind of swallows everything else in the union.

If any gives us the opposite of what we want, maybe the opposite of any (never) will give us exactly what we’re looking for?

ts
let myValue: string | number | never
      
let myValue: string | numberTry

Great! Let’s go back to our ConstructorArg<C> type and add this in

ts
type ConstructorArg<C> = C extends {
  new (arg: infer A, ...args: any[]): any
}
  ? A
  : neverTry

And we’re done!

ts
let dateFirst: ConstructorArg<typeof Date>
       
let dateFirst: string | number | Date
let promiseFirst: ConstructorArg<typeof Promise>
         
let promiseFirst: (resolve: (value: unknown) => void, reject: (reason?: any) => void) => void
 
let webpackCfg: ConstructorArg<typeof WebpackCompiler>
        
let webpackCfg: {
    amd?: false | {
        [index: string]: any;
    };
    bail?: boolean;
    cache?: boolean | {
        maxGenerations?: number;
        type: "memory";
    };
    context?: string;
    ... 20 more ...;
    watch?: boolean;
}Try

Awesome! Now if we go back to the original thing we were trying to do, we get some improved type safety

diff
-const cfg = {
+const cfg: ConstructorArg<typeof WebpackCompiler> = {
  entry: "src/index.ts",
  wutch: true, // SPELLING ERROR!!
}

ts
type ConstructorArg<C> = C extends {
  new (arg: infer A, ...args: any[]): any
}
  ? A
  : never
 
const cfg: ConstructorArg<typeof WebpackCompiler> = {
  entry: "src/index.ts",
  wutch: true, // SPELLING ERROR!!
Object literal may only specify known properties, but 'wutch' does not exist in type '{ amd?: false | { [index: string]: any; } | undefined; bail?: boolean | undefined; cache?: boolean | { maxGenerations?: number | undefined; type: "memory"; } | undefined; context?: string | undefined; ... 20 more ...; watch?: boolean | undefined; }'. Did you mean to write 'watch'?2561Object literal may only specify known properties, but 'wutch' does not exist in type '{ amd?: false | { [index: string]: any; } | undefined; bail?: boolean | undefined; cache?: boolean | { maxGenerations?: number | undefined; type: "memory"; } | undefined; context?: string | undefined; ... 20 more ...; watch?: boolean | undefined; }'. Did you mean to write 'watch'?
}
try {
  const compiler = new WebpackCompiler(cfg)
} catch (err) {
  throw new Error(
    `Problem compiling with config\n${JSON.stringify(
      cfg,
      null,
      "  "
    )}`
  )
}Try

Success!