es6/operators/max.js

import { reduce } from './reduce';
/**
 * The Max operator operates on an Observable that emits numbers (or items that can be compared with a provided function),
 * and when source Observable completes it emits a single item: the item with the largest value.
 *
 * <img src="./img/max.png" width="100%">
 *
 * @example <caption>Get the maximal value of a series of numbers</caption>
 * Rx.Observable.of(5, 4, 7, 2, 8)
 *   .max()
 *   .subscribe(x => console.log(x)); // -> 8
 *
 * @example <caption>Use a comparer function to get the maximal item</caption>
 * interface Person {
 *   age: number,
 *   name: string
 * }
 * Observable.of<Person>({age: 7, name: 'Foo'},
 *                       {age: 5, name: 'Bar'},
 *                       {age: 9, name: 'Beer'})
 *           .max<Person>((a: Person, b: Person) => a.age < b.age ? -1 : 1)
 *           .subscribe((x: Person) => console.log(x.name)); // -> 'Beer'
 * }
 *
 * @see {@link min}
 *
 * @param {Function} [comparer] - Optional comparer function that it will use instead of its default to compare the
 * value of two items.
 * @return {Observable} An Observable that emits item with the largest value.
 * @method max
 * @owner Observable
 */
export function max(comparer) {
    const max = (typeof comparer === 'function')
        ? (x, y) => comparer(x, y) > 0 ? x : y
        : (x, y) => x > y ? x : y;
    return reduce(max);
}
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