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TS: isTruthy
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| // copyright 2023 Zach Hardesty | |
| // want to check this out in the TypeScript playground? | |
| // visit the following link to automatically see the latest version! | |
| // https://www.typescriptlang.org/play?jsx=0#gist/b1d6650475c0b30dc57eebe13d20ae37 | |
| /** | |
| * Type guard helper for checking if a value is truthy. mostly for use with | |
| * {@link Array.prototype.filter} to clean up array types | |
| * | |
| * @example | |
| * const foo = true | |
| * const bar = false | |
| * const baz = [foo && "string1", bar && "string2"] | |
| * console.log(baz.filter(isTruthy)) | |
| * // => ["string1"] | |
| * // boolean is removed from type of baz and it becomes string[] | |
| * | |
| * @param input - Value to check | |
| * @see https://stackoverflow.com/a/19839953/5299167 for a list of falsey values | |
| */ | |
| function isTruthy<T>( | |
| input: T, | |
| ): input is T extends undefined | null | false | "" | 0 | 0n ? never : T { | |
| return !!input; | |
| } | |
| const x0_0 = ([true, false] as const).filter(isTruthy); | |
| type T0_0 = Expect<typeof x0_0, true[]>; | |
| const x1_0 = ([1, 0, 0.0, -0, 0x0] as const).filter(isTruthy); | |
| type T1_0 = Expect<typeof x1_0, 1[]>; | |
| const x2_0 = ([{}, undefined, null] as const).filter(isTruthy); | |
| type T2_0 = Expect<typeof x2_0, {}[]>; | |
| const x3_0 = (["truthy", "", ``] as const).filter(isTruthy); | |
| type T3_0 = Expect<typeof x3_0, "truthy"[]>; | |
| const x4_0 = ([1n, 0n, 0x0n] as const).filter(isTruthy); | |
| type T4_0 = Expect<typeof x4_0, 1n[]>; | |
| const x5_0 = ([0, false] as const).filter(isTruthy); | |
| type T5_0 = Expect<typeof x5_0, never[]>; | |
| const x6_0 = ([true, NaN, Number.NaN] as const).filter(isTruthy); | |
| // @ts-expect-error - no literal type for NaN, so impossible to remove `number` from result type | |
| type T6_0 = Expect<typeof x6_0, true[]>; | |
| const x7_0 = ([] as never[]).filter(isTruthy); | |
| type T7_0 = Expect<typeof x7_0, never[]>; | |
| const v010 = Math.random() > 0.5 ? "a" : undefined; | |
| if (isTruthy(v010)) { | |
| type T010 = Expect<typeof v010, "a">; | |
| } else { | |
| type T010 = Expect<typeof v010, undefined>; | |
| } | |
| const v020 = Math.random() > 0.5 ? "a" : false; | |
| if (isTruthy(v020)) { | |
| type T020 = Expect<typeof v020, "a">; | |
| } else { | |
| type T020 = Expect<typeof v020, false>; | |
| } | |
| const v030 = Math.random() > 0.5 ? "a" : Math.random() > 0.5 ? "" : false; | |
| if (isTruthy(v030)) { | |
| type T030 = Expect<typeof v030, "a">; | |
| } else { | |
| type T030 = Expect<typeof v030, false | "">; | |
| } | |
| type Result = "ACK" | "ERR" | ""; | |
| declare const result: Result; | |
| if (result) { | |
| console.log("🚀 ~ result:", result); | |
| // handle that a result was received | |
| } else { | |
| console.log("🚀 ~ result:", result); | |
| // couldn't get a result | |
| } | |
| /** @see https://gist.github.com/zachhardesty7/1b5b6f6de328f1bb7c78cbe96ba5c720 */ | |
| type Expect< | |
| TInput extends TIntersect, | |
| TExpected extends TIntersect, | |
| TIntersect = TExpected & TInput, | |
| > = TInput; |
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