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Merged
merged 6 commits into from
Aug 16, 2023
+70 −3
Merged

Make assert, truthy and falsy typeguards #3233

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f106e37
Make `assert`, `truthy` and `falsy` typeguards
ZachHaber Aug 14, 2023
8c6696f
remove nulls from tests to fix lint error
ZachHaber Aug 14, 2023
35083a9
remove -0 as it just resolves to 0
ZachHaber Aug 14, 2023
04b0f49
Improve types for number and string falsy values
ZachHaber Aug 16, 2023
58274c1
Remove explicit mention of typescript
ZachHaber Aug 16, 2023
c9ec4b1
Fix other notes to remove the explicit typescript callout
ZachHaber Aug 16, 2023
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28 changes: 28 additions & 0 deletions test-types/import-in-cts/assertions-as-type-guards.cts
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,15 @@ import {expectType} from 'tsd';
type Expected = {foo: 'bar'};
const expected: Expected = {foo: 'bar'};

test('assert', t => {
const actual = expected as Expected | undefined;
if (t.truthy(actual)) {
expectType<Expected>(actual);
} else {
expectType<undefined>(actual);
}
});

test('deepEqual', t => {
const actual: unknown = {};
if (t.deepEqual(actual, expected)) {
Expand Down Expand Up @@ -32,9 +41,28 @@ test('false', t => {
}
});

test('falsy', t => {
type Actual = Expected | undefined | false | 0 | '' | 0n;
const actual = undefined as Actual;
if (t.falsy(actual)) {
expectType<Exclude<Actual, Expected>>(actual);
} else {
expectType<Expected>(actual);
}
});

test('true', t => {
const actual: unknown = false;
if (t.true(actual)) {
expectType<true>(actual);
}
});

test('truthy', t => {
const actual = expected as Expected | undefined;
if (t.truthy(actual)) {
expectType<Expected>(actual);
} else {
expectType<undefined>(actual);
}
});
28 changes: 28 additions & 0 deletions test-types/module/assertions-as-type-guards.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,15 @@ import test from '../../entrypoints/main.mjs';
type Expected = {foo: 'bar'};
const expected: Expected = {foo: 'bar'};

test('assert', t => {
const actual = expected as Expected | undefined;
if (t.truthy(actual)) {
expectType<Expected>(actual);
} else {
expectType<undefined>(actual);
}
});

test('deepEqual', t => {
const actual: unknown = {};
if (t.deepEqual(actual, expected)) {
Expand Down Expand Up @@ -33,9 +42,28 @@ test('false', t => {
}
});

test('falsy', t => {
type Actual = Expected | undefined | false | 0 | '' | 0n;
const actual = undefined as Actual;
if (t.falsy(actual)) {
expectType<Exclude<Actual, Expected>>(actual);
} else {
expectType<Expected>(actual);
}
});

test('true', t => {
const actual: unknown = false;
if (t.true(actual)) {
expectType<true>(actual);
}
});

test('truthy', t => {
const actual = expected as Expected | undefined;
if (t.truthy(actual)) {
expectType<Expected>(actual);
} else {
expectType<undefined>(actual);
}
});
8 changes: 5 additions & 3 deletions types/assertions.d.cts
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Original file line number Diff line number Diff line change
Expand Up @@ -125,12 +125,14 @@ export type Assertions = {
truthy: TruthyAssertion;
};

type Falsy = false | 0 | 0n | '' | null | undefined;

export type AssertAssertion = {
/**
* Assert that `actual` is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy), returning a boolean
* indicating whether the assertion passed.
*/
(actual: any, message?: string): boolean;
<T>(actual: T, message?: string): actual is Exclude<T, Falsy>;

/** Skip this assertion. */
skip(actual: any, message?: string): void;
Expand Down Expand Up @@ -192,7 +194,7 @@ export type FalsyAssertion = {
* Assert that `actual` is [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy), returning a boolean
* indicating whether the assertion passed.
*/
(actual: any, message?: string): boolean;
<T>(actual: T, message?: string): actual is T extends Falsy ? T :never;
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I don't think never is correct here.

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@ZachHaber ZachHaber Aug 15, 2023 •
edited
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I don't think we can use anything besides never there and have it work correctly. The only things that typescript doesn't complain about using there are never and any or a subset of the T type. If you make it any then falsy(true) thinks it's falsy.

This is also the exact opposite condition I setup in truthy and assert:
actual is Exclude<T,Falsy> where Exclude is Exclude<T,U> = T extends U ? never : T becomes actual is T extends Falsy ? never : T which is the opposite condition of the falsy assertion.

I included the test-types with both the if case and the else case so you can see the types as they work. For further showcasing of the types, I setup a quick Playground example if you want to try changing what the assertions are.

Playing around with it, the current conditions don't work well for number and string primitives. Typescript seems to not recognize that 0 is a subset of number unless you use an as const expression with it. Similar results for an empty string ('') but even more wacky as '' will make typescript think it is truthy with a value of '', but '' as const makes typescript recognize it as falsy. Typescript also doesn't separate out 0 or '' from their primitive values.

So far, I think I'm having the most accurate approach using (playground link)

function falsy<T>(actual: T): actual is T extends Exclude<T, Falsy>? never : T {
    return !actual;
};
function truthy<T>(actual: T): actual is Exclude<T, Falsy> {
    return !!actual;
};

In which case the only remaining issue is that for a type of number or string, typescript doesn't think that they could ever be falsy.

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I don't think we can use anything besides never there and have it work correctly.

You're right. I read the signature incorrectly.

So far, I think I'm having the most accurate approach using (playground link)

👍

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With your suggestion of multiple overloads, that partially works, but runs into an issue where union types would fall back to the base behavior of the generic rather than the behavior of the overload.

I think I've cracked it for falsy, but I can't get the same to work for truthy as Typescript has no way to just invert a type. playground - As you can see from the screenshot, falsy works great, but truthy is missing 0 in the else clause, and I don't think there's really anything we can do about it...
image

type FalsyValue = false | 0 | 0n | '' | null | undefined;
type Falsy<T> = T extends Exclude<T, FalsyValue> ? (T extends number | string | bigint ? T & FalsyValue : never) : T;

function falsy<T>(actual: T): actual is Falsy<T> {
    return !actual;
};
function truthy<T>(actual: T): actual is T extends Falsy<T> ? never : T {
    return !!actual;
};

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This looks good 👍

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but truthy is missing 0 in the else clause, and I don't think there's really anything we can do about it...

I would mention this in the doc comments.

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Does it work to have specific overloads?

Suggested change
<T>(actual: T, message?: string): actual is T extends Falsy ? T :never;
(actual: 0): true;
(actual: ''): true;
<T>(actual: T, message?: string): actual is T extends Falsy ? T :never;

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Suggested change
<T>(actual: T, message?: string): actual is T extends Falsy ? T :never;
<T>(actual: T, message?: string): actual is T extends Falsy ? T : never;


/** Skip this assertion. */
skip(actual: any, message?: string): void;
Expand Down Expand Up @@ -337,7 +339,7 @@ export type TruthyAssertion = {
* Assert that `actual` is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy), returning a boolean
* indicating whether the assertion passed.
*/
(actual: any, message?: string): boolean;
<T>(actual: T, message?: string): actual is Exclude<T, Falsy>;

/** Skip this assertion. */
skip(actual: any, message?: string): void;
Expand Down