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+import { OperatorFunction, ObservableInputTuple } from '../types';
+import { operate } from '../util/lift';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+import { innerFrom } from '../observable/innerFrom';
+import { identity } from '../util/identity';
+import { noop } from '../util/noop';
+import { popResultSelector } from '../util/args';
+
+export function withLatestFrom<T, O extends unknown[]>(...inputs: [...ObservableInputTuple<O>]): OperatorFunction<T, [T, ...O]>;
+
+export function withLatestFrom<T, O extends unknown[], R>(
+  ...inputs: [...ObservableInputTuple<O>, (...value: [T, ...O]) => R]
+): OperatorFunction<T, R>;
+
+/**
+ * Combines the source Observable with other Observables to create an Observable
+ * whose values are calculated from the latest values of each, only when the
+ * source emits.
+ *
+ * <span class="informal">Whenever the source Observable emits a value, it
+ * computes a formula using that value plus the latest values from other input
+ * Observables, then emits the output of that formula.</span>
+ *
+ * ![](withLatestFrom.png)
+ *
+ * `withLatestFrom` combines each value from the source Observable (the
+ * instance) with the latest values from the other input Observables only when
+ * the source emits a value, optionally using a `project` function to determine
+ * the value to be emitted on the output Observable. All input Observables must
+ * emit at least one value before the output Observable will emit a value.
+ *
+ * ## Example
+ *
+ * On every click event, emit an array with the latest timer event plus the click event
+ *
+ * ```ts
+ * import { fromEvent, interval, withLatestFrom } from 'rxjs';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const timer = interval(1000);
+ * const result = clicks.pipe(withLatestFrom(timer));
+ * result.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link combineLatest}
+ *
+ * @param inputs An input Observable to combine with the source Observable. More
+ * than one input Observables may be given as argument. If the last parameter is
+ * a function, it will be used as a projection function for combining values
+ * together. When the function is called, it receives all values in order of the
+ * Observables passed, where the first parameter is a value from the source
+ * Observable. (e.g.
+ * `a.pipe(withLatestFrom(b, c), map(([a1, b1, c1]) => a1 + b1 + c1))`). If this
+ * is not passed, arrays will be emitted on the output Observable.
+ * @return A function that returns an Observable of projected values from the
+ * most recent values from each input Observable, or an array of the most
+ * recent values from each input Observable.
+ */
+export function withLatestFrom<T, R>(...inputs: any[]): OperatorFunction<T, R | any[]> {
+  const project = popResultSelector(inputs) as ((...args: any[]) => R) | undefined;
+
+  return operate((source, subscriber) => {
+    const len = inputs.length;
+    const otherValues = new Array(len);
+    // An array of whether or not the other sources have emitted. Matched with them by index.
+    // TODO: At somepoint, we should investigate the performance implications here, and look
+    // into using a `Set()` and checking the `size` to see if we're ready.
+    let hasValue = inputs.map(() => false);
+    // Flipped true when we have at least one value from all other sources and
+    // we are ready to start emitting values.
+    let ready = false;
+
+    // Other sources. Note that here we are not checking `subscriber.closed`,
+    // this causes all inputs to be subscribed to, even if nothing can be emitted
+    // from them. This is an important distinction because subscription constitutes
+    // a side-effect.
+    for (let i = 0; i < len; i++) {
+      innerFrom(inputs[i]).subscribe(
+        createOperatorSubscriber(
+          subscriber,
+          (value) => {
+            otherValues[i] = value;
+            if (!ready && !hasValue[i]) {
+              // If we're not ready yet, flag to show this observable has emitted.
+              hasValue[i] = true;
+              // Intentionally terse code.
+              // If all of our other observables have emitted, set `ready` to `true`,
+              // so we know we can start emitting values, then clean up the `hasValue` array,
+              // because we don't need it anymore.
+              (ready = hasValue.every(identity)) && (hasValue = null!);
+            }
+          },
+          // Completing one of the other sources has
+          // no bearing on the completion of our result.
+          noop
+        )
+      );
+    }
+
+    // Source subscription
+    source.subscribe(
+      createOperatorSubscriber(subscriber, (value) => {
+        if (ready) {
+          // We have at least one value from the other sources. Go ahead and emit.
+          const values = [value, ...otherValues];
+          subscriber.next(project ? project(...values) : values);
+        }
+      })
+    );
+  });
+}