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<div class="content" data-ice="content"><h1 data-ice="title">lib/waterfall.js</h1>
<pre class="source-code line-number raw-source-code"><code class="prettyprint linenums" data-ice="content">import isArray from &apos;lodash/isArray&apos;;
import noop from &apos;lodash/noop&apos;;
import once from &apos;./internal/once&apos;;
import rest from &apos;lodash/rest&apos;;

import onlyOnce from &apos;./internal/onlyOnce&apos;;

/**
 * Runs the `tasks` array of functions in series, each passing their results to
 * the next in the array. However, if any of the `tasks` pass an error to their
 * own callback, the next function is not executed, and the main `callback` is
 * immediately called with the error.
 *
 * @name waterfall
 * @static
 * @memberOf async
 * @category Control Flow
 * @param {Array} tasks - An array of functions to run, each function is passed
 * a `callback(err, result1, result2, ...)` it must call on completion. The
 * first argument is an error (which can be `null`) and any further arguments
 * will be passed as arguments in order to the next task.
 * @param {Function} [callback] - An optional callback to run once all the
 * functions have completed. This will be passed the results of the last task&apos;s
 * callback. Invoked with (err, [results]).
 * @returns undefined
 * @example
 *
 * async.waterfall([
 *     function(callback) {
 *         callback(null, &apos;one&apos;, &apos;two&apos;);
 *     },
 *     function(arg1, arg2, callback) {
 *         // arg1 now equals &apos;one&apos; and arg2 now equals &apos;two&apos;
 *         callback(null, &apos;three&apos;);
 *     },
 *     function(arg1, callback) {
 *         // arg1 now equals &apos;three&apos;
 *         callback(null, &apos;done&apos;);
 *     }
 * ], function (err, result) {
 *     // result now equals &apos;done&apos;
 * });
 *
 * // Or, with named functions:
 * async.waterfall([
 *     myFirstFunction,
 *     mySecondFunction,
 *     myLastFunction,
 * ], function (err, result) {
 *     // result now equals &apos;done&apos;
 * });
 * function myFirstFunction(callback) {
 *     callback(null, &apos;one&apos;, &apos;two&apos;);
 * }
 * function mySecondFunction(arg1, arg2, callback) {
 *     // arg1 now equals &apos;one&apos; and arg2 now equals &apos;two&apos;
 *     callback(null, &apos;three&apos;);
 * }
 * function myLastFunction(arg1, callback) {
 *     // arg1 now equals &apos;three&apos;
 *     callback(null, &apos;done&apos;);
 * }
 */
export default  function(tasks, callback) {
    callback = once(callback || noop);
    if (!isArray(tasks)) return callback(new Error(&apos;First argument to waterfall must be an array of functions&apos;));
    if (!tasks.length) return callback();
    var taskIndex = 0;

    function nextTask(args) {
        if (taskIndex === tasks.length) {
            return callback.apply(null, [null].concat(args));
        }

        var taskCallback = onlyOnce(rest(function(err, args) {
            if (err) {
                return callback.apply(null, [err].concat(args));
            }
            nextTask(args);
        }));

        args.push(taskCallback);

        var task = tasks[taskIndex++];
        task.apply(null, args);
    }

    nextTask([]);
}
</code></pre>

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