ix/iterable/single.mjs

The Interactive Extensions for JavaScript

/**
 * Returns the only element of an iterable sequence that matches the predicate if specified,
 * or undefined if no such element exists; this method reports an exception if there is more
 * than one element in the iterable sequence.
 *
 * @template T The type of the elements in the source sequence.
 * @param {AsyncIterable<T>} source Source iterable sequence.
 * @param {OptionalFindOptions<T>} [options] The optional options which includes a predicate for filtering,
 * and thisArg for predicate binding.
 * @returns {(T | undefined)} The single element in the iterable sequence that satisfies
 * the condition in the predicate, or undefined if no such element exists.
 */
export function single(source, options) {
    const { ['thisArg']: thisArg, ['predicate']: predicate = () => true } = options || {};
    let result;
    let hasResult = false;
    let i = 0;
    for (const item of source) {
        if (hasResult && predicate.call(thisArg, item, i++)) {
            throw new Error('More than one element was found');
        }
        if (predicate.call(thisArg, item, i++)) {
            result = item;
            hasResult = true;
        }
    }
    return result;
}

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