Allow intersection type guards for multiple parameters

[SLaks]

Search Terms

type guard multiple parameters

Suggestion

I'd like to write a type guard that takes two parameters and changes the type of both of them.

Use Cases

My specific use case is to try to make the following pattern (somewhat) more type-safe:

class Foo<TFeature, TOther> {
 // If featureCtor is null, TFeature will never be used.
 constructor(private readonly featureCtor: { new(): TFeature } | null) { }

 isFeature(thing: any): thing is TFeature {
 return !!this.featureCtor && thing instanceof this.featureCtor;
 }

 bar(thing: TFeature|TOther) {
 if (this.isFeature(thing)) {
 // Type guard should prove that this.featureCtor is not null
 new this.featureCtor();
 } else {
 // Type guard should prove this
 const x: TOther = thing;
 }
 } 
}

Examples

isFeature(thing: any, ctor: { new(): TFeature } | null): (thing is TFeature)&(ctor is { new(): TFeature }) {
 return !!this.featureCtor && thing instanceof this.featureCtor;
}

It would be even nicer to allow type guards to operate on readonly fields, so I wouldn't need to pass this.featureCtor as a parameter.

I also tried

constructor(private readonly featureCtor: TFeature extends never ? null : { new(): TFeature }) { }

But that didn't work.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

[jyrodrigues]

Any news on this?

[quisido]

Seconding a need for this:

const isValidPair = (x: null | string, y: null | string): x is string & y is null {
 return typeof x === 'string' && y === null;
};

if (isValidPair(var1, var2)) {
 // know var1 is string
 // know var2 is null
}

[kayahr]

This would be useful for processing overloaded function parameters where it is enough to check one parameter to assume the type of the other parameter:

function nonsense(a: number, b: number): number;
function nonsense(a: string, b: string): string;
function nonsense(a: number | string, b: number | string): number | string {
 const isNumbers = (a: number | string, b: number | string): a is number & b is number 
 => typeof a === "number";

 if (isNumbers(a, b)) {
 return a * 10 + b * 100;
 } else {
 return a.length + b.length;
 }
}

[Fnxxxxo]

This is also convenient and meaningful for builders.

class Builder {
 private fixtureA: string | null = null
 private fixtureB: string | null = null
 // ... any other C, D, E ...
 public build() {
 if (this.ofTypeAB()) {
 return {}
 }
 if (this.ofTypeCDA()) {
 return {}
 }
 }
}

[m5r]

Hi, any updates?

[mordv]

I think it would be useful for otherwise annoying ref.current checking in react

const refsValued = <T extends unknown>(...refs: { current: T | undefined | null }[]): ...refs is { current: T }[] =>
 !refs.find((r) => !r.current);

[avalanche1]

bump!

[helmturner]

Would be great for syntax trees, as well.

I actually assumed this was possible, and tried the following syntaxes before finding this issue.

This syntax seems natural:

import {convert} from 'unist-util-is'
import type {Paragraph, Root} from 'mdast'

export const isNonNestedParagraph = convert<Paragraph>(
	(node, _, parent): node is Paragraph & parent is Root =>
		!!(node.type === "paragraph" && parent?.type === "root")
);

But it logically follows that a destructuring pattern could be used, as well.

Something like the following:

import {convert} from 'unist-util-is'
import type {Paragraph, Root} from 'mdast'

export const isNonNestedParagraph = convert<Paragraph>(
	(node, _, parent): [node, parent] is [Paragraph, Root] =>
		!!(node.type === "paragraph" && parent?.type === "root")
);

or:

import {convert} from 'unist-util-is'
import type {Paragraph, Root} from 'mdast'

export const isNonNestedParagraph = convert<Paragraph>(
	(node, _, parent): {node, parent} is {node: Paragraph, parent: Root} =>
		!!(node.type === "paragraph" && parent?.type === "root")
);

[joyt]

Would be really useful to have this. I wanted something like then when trying to type a deepEqual function:

// Would like assert that BOTH parameters are the intersection of their respective types
export function deepEqual<A, B>(a: A, b: B): (a is A & B) && (b is A & B) {...}