feat: tap once

Author: endlacer

docs/src/content/docs/utilities/Operators/tap-once.md

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+````markdown
+---
+title: tapOnce / tapOnceOnFirstTruthy
+description: Standalone RxJS operators for executing functions conditionally on emitted values.
+entryPoint: ngxtension/tap-once
+badge: stable
+contributors: ['andreas-dorner']
+---
+
+## Import
+
+```typescript
+import { tapOnce, tapOnceOnFirstTruthy } from 'ngxtension/tap-once';
+```
+````
+
+## Usage
+
+### tapOnce
+
+Executes the provided function only once when the value at the specified index is emitted.
+
+```typescript
+import { from } from 'rxjs';
+import { tapOnce } from 'ngxtension/tap-once';
+
+const in$ = from([1, 2, 3, 4, 5]);
+const out$ = in$.pipe(tapOnce((value) => console.log(value), 2));
+
+out$.subscribe(); // logs: 3
+```
+
+#### Parameters
+
+- `tapFn`: Function to execute on the value at the specified index.
+- `tapIndex`: Index at which to execute the function (default is 0).
+
+### tapOnceOnFirstTruthy
+
+Executes the provided function only once when the first truthy value is emitted.
+
+```typescript
+import { from } from 'rxjs';
+import { tapOnceOnFirstTruthy } from 'ngxtension/tap-once';
+
+const in$ = from([0, null, false, 3, 4, 5]);
+const out$ = in$.pipe(tapOnceOnFirstTruthy((value) => console.log(value)));
+
+out$.subscribe(); // logs: 3
+```
+
+#### Parameters
+
+- `tapFn`: Function to execute on the first truthy value.
+
+## API
+
+### tapOnce
+
+- `tapFn: (t: T) => void`
+- `tapIndex: number = 0`
+
+### tapOnceOnFirstTruthy
+
+- `tapFn: (t: T) => void`
+
+### Validation
+
+- Throws an error if `tapIndex` is negative.
+
+```
+
+```

libs/ngxtension/tapOnce/README.md

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+# ngxtension/tap-once
+
+Secondary entry point of `ngxtension`. It can be used by importing from `ngxtension/tap-once`.

libs/ngxtension/tapOnce/ng-package.json

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+{
+	"lib": {
+		"entryFile": "src/index.ts"
+	}
+}

libs/ngxtension/tapOnce/project.json

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+{
+	"name": "ngxtension/tap-once",
+	"$schema": "../../../node_modules/nx/schemas/project-schema.json",
+	"projectType": "library",
+	"sourceRoot": "libs/ngxtension/tap-once/src",
+	"targets": {
+		"test": {
+			"executor": "@nx/jest:jest",
+			"outputs": ["{workspaceRoot}/coverage/{projectRoot}"],
+			"options": {
+				"jestConfig": "libs/ngxtension/jest.config.ts",
+				"testPathPattern": ["tap-once"]
+			}
+		},
+		"lint": {
+			"executor": "@nx/eslint:lint",
+			"outputs": ["{options.outputFile}"]
+		}
+	}
+}

libs/ngxtension/tapOnce/src/index.ts

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+export * from './tap-once';

libs/ngxtension/tapOnce/src/tap-once.spec.ts

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+import { from, toArray } from 'rxjs';
+import { tapOnce, tapOnceOnFirstTruthy } from './tap-once';
+
+describe(tapOnce.name, () => {
+	it('should execute the function only once at the specified index', (done) => {
+		const tapFn = jest.fn();
+		const in$ = from([1, 2, 3, 4, 5]);
+		const out$ = in$.pipe(tapOnce(tapFn, 2));
+
+		out$.pipe(toArray()).subscribe((r) => {
+			expect(r).toEqual([1, 2, 3, 4, 5]);
+			expect(tapFn).toHaveBeenCalledTimes(1);
+			expect(tapFn).toHaveBeenCalledWith(3);
+			done();
+		});
+	});
+
+	it('should execute the function only once at the default index 0', (done) => {
+		const tapFn = jest.fn();
+		const in$ = from([1, 2, 3, 4, 5]);
+		const out$ = in$.pipe(tapOnce(tapFn));
+
+		out$.pipe(toArray()).subscribe((r) => {
+			expect(r).toEqual([1, 2, 3, 4, 5]);
+			expect(tapFn).toHaveBeenCalledTimes(1);
+			expect(tapFn).toHaveBeenCalledWith(1);
+			done();
+		});
+	});
+
+	it('should throw an error if tapIndex is negative', () => {
+		expect(() => tapOnce(() => {}, -1)).toThrow(
+			'tapIndex must be a non-negative integer',
+		);
+	});
+});
+
+describe(tapOnceOnFirstTruthy.name, () => {
+	it('should execute the function only once on the first truthy value', (done) => {
+		const tapFn = jest.fn();
+		const in$ = from([0, null, false, 3, 4, 5]);
+		const out$ = in$.pipe(tapOnceOnFirstTruthy(tapFn));
+
+		out$.pipe(toArray()).subscribe((r) => {
+			expect(r).toEqual([0, null, false, 3, 4, 5]);
+			expect(tapFn).toHaveBeenCalledTimes(1);
+			expect(tapFn).toHaveBeenCalledWith(3);
+			done();
+		});
+	});
+
+	it('should not execute the function if there are no truthy values', (done) => {
+		const tapFn = jest.fn();
+		const in$ = from([0, null, false, undefined]);
+		const out$ = in$.pipe(tapOnceOnFirstTruthy(tapFn));
+
+		out$.pipe(toArray()).subscribe((r) => {
+			expect(r).toEqual([0, null, false, undefined]);
+			expect(tapFn).not.toHaveBeenCalled();
+			done();
+		});
+	});
+
+	it('should execute the function only once even if there are multiple truthy values', (done) => {
+		const tapFn = jest.fn();
+		const in$ = from([1, 2, 3, 4, 5]);
+		const out$ = in$.pipe(tapOnceOnFirstTruthy(tapFn));
+
+		out$.pipe(toArray()).subscribe((r) => {
+			expect(r).toEqual([1, 2, 3, 4, 5]);
+			expect(tapFn).toHaveBeenCalledTimes(1);
+			expect(tapFn).toHaveBeenCalledWith(1);
+			done();
+		});
+	});
+});

libs/ngxtension/tapOnce/src/tap-once.ts

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+import { MonoTypeOperatorFunction, concatMap, of, type Observable } from 'rxjs';
+import { tap } from 'rxjs/operators';
+
+/**
+ * Executes the provided function only once when the first truthy value is emitted.
+ * @param tapFn - Function to execute on the first truthy value.
+ * @returns MonoTypeOperatorFunction
+ */
+export function tapOnceOnFirstTruthy<T>(
+	tapFn: (t: T) => void,
+): MonoTypeOperatorFunction<T> {
+	let firstTruthy = true;
+	return (source$: Observable<T>) =>
+		source$.pipe(
+			tap((value) => {
+				if (firstTruthy && !!value) {
+					tapFn(value);
+					firstTruthy = false;
+				}
+			}),
+		);
+}
+
+/**
+ * Executes the provided function only once when the value at the specified index is emitted.
+ * @param tapFn - Function to execute on the value at the specified index.
+ * @param tapIndex - Index at which to execute the function (default is 0).
+ * @returns MonoTypeOperatorFunction
+ */
+export function tapOnce<T>(
+	tapFn: (t: T) => void,
+	tapIndex = 0,
+): MonoTypeOperatorFunction<T> {
+	if (tapIndex < 0) {
+		throw new Error('tapIndex must be a non-negative integer');
+	}
+	return (source$: Observable<T>) =>
+		source$.pipe(
+			concatMap((value, index) => {
+				if (index === tapIndex) {
+					tapFn(value);
+				}
+				return of(value);
+			}),
+		);
+}