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import eachOfSeries from './eachOfSeries.js'
import once from './internal/once.js'
import wrapAsync from './internal/wrapAsync.js'
import awaitify from './internal/awaitify.js'
/**
* Reduces `coll` into a single value using an async `iteratee` to return each
* successive step. `memo` is the initial state of the reduction. This function
* only operates in series.
*
* For performance reasons, it may make sense to split a call to this function
* into a parallel map, and then use the normal `Array.prototype.reduce` on the
* results. This function is for situations where each step in the reduction
* needs to be async; if you can get the data before reducing it, then it's
* probably a good idea to do so.
*
* @name reduce
* @static
* @memberOf module:Collections
* @method
* @alias inject
* @alias foldl
* @category Collection
* @param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
* @param {*} memo - The initial state of the reduction.
* @param {AsyncFunction} iteratee - A function applied to each item in the
* array to produce the next step in the reduction.
* The `iteratee` should complete with the next state of the reduction.
* If the iteratee completes with an error, the reduction is stopped and the
* main `callback` is immediately called with the error.
* Invoked with (memo, item, callback).
* @param {Function} [callback] - A callback which is called after all the
* `iteratee` functions have finished. Result is the reduced value. Invoked with
* (err, result).
* @returns {Promise} a promise, if no callback is passed
* @example
*
* // file1.txt is a file that is 1000 bytes in size
* // file2.txt is a file that is 2000 bytes in size
* // file3.txt is a file that is 3000 bytes in size
* // file4.txt does not exist
*
* const fileList = ['file1.txt','file2.txt','file3.txt'];
* const withMissingFileList = ['file1.txt','file2.txt','file3.txt', 'file4.txt'];
*
* // asynchronous function that computes the file size in bytes
* // file size is added to the memoized value, then returned
* function getFileSizeInBytes(memo, file, callback) {
* fs.stat(file, function(err, stat) {
* if (err) {
* return callback(err);
* }
* callback(null, memo + stat.size);
* });
* }
*
* // Using callbacks
* async.reduce(fileList, 0, getFileSizeInBytes, function(err, result) {
* if (err) {
* console.log(err);
* } else {
* console.log(result);
* // 6000
* // which is the sum of the file sizes of the three files
* }
* });
*
* // Error Handling
* async.reduce(withMissingFileList, 0, getFileSizeInBytes, function(err, result) {
* if (err) {
* console.log(err);
* // [ Error: ENOENT: no such file or directory ]
* } else {
* console.log(result);
* }
* });
*
* // Using Promises
* async.reduce(fileList, 0, getFileSizeInBytes)
* .then( result => {
* console.log(result);
* // 6000
* // which is the sum of the file sizes of the three files
* }).catch( err => {
* console.log(err);
* });
*
* // Error Handling
* async.reduce(withMissingFileList, 0, getFileSizeInBytes)
* .then( result => {
* console.log(result);
* }).catch( err => {
* console.log(err);
* // [ Error: ENOENT: no such file or directory ]
* });
*
* // Using async/await
* async () => {
* try {
* let result = await async.reduce(fileList, 0, getFileSizeInBytes);
* console.log(result);
* // 6000
* // which is the sum of the file sizes of the three files
* }
* catch (err) {
* console.log(err);
* }
* }
*
* // Error Handling
* async () => {
* try {
* let result = await async.reduce(withMissingFileList, 0, getFileSizeInBytes);
* console.log(result);
* }
* catch (err) {
* console.log(err);
* // [ Error: ENOENT: no such file or directory ]
* }
* }
*
*/
function reduce(coll, memo, iteratee, callback) {
callback = once(callback);
var _iteratee = wrapAsync(iteratee);
return eachOfSeries(coll, (x, i, iterCb) => {
_iteratee(memo, x, (err, v) => {
memo = v;
iterCb(err);
});
}, err => callback(err, memo));
}
export default awaitify(reduce, 4)
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