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<h1 class="page-title">transform.js</h1>
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<pre class="prettyprint source linenums"><code>import eachOf from './eachOf.js'
import once from './internal/once.js'
import wrapAsync from './internal/wrapAsync.js'
import { promiseCallback, PROMISE_SYMBOL } from './internal/promiseCallback.js'
/**
* A relative of `reduce`. Takes an Object or Array, and iterates over each
* element in parallel, each step potentially mutating an `accumulator` value.
* The type of the accumulator defaults to the type of collection passed in.
*
* @name transform
* @static
* @memberOf module:Collections
* @method
* @category Collection
* @param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
* @param {*} [accumulator] - The initial state of the transform. If omitted,
* it will default to an empty Object or Array, depending on the type of `coll`
* @param {AsyncFunction} iteratee - A function applied to each item in the
* collection that potentially modifies the accumulator.
* Invoked with (accumulator, item, key, callback).
* @param {Function} [callback] - A callback which is called after all the
* `iteratee` functions have finished. Result is the transformed accumulator.
* Invoked with (err, result).
* @returns {Promise} a promise, if no callback provided
* @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
*
* // helper function that returns human-readable size format from bytes
* function formatBytes(bytes, decimals = 2) {
* // implementation not included for brevity
* return humanReadbleFilesize;
* }
*
* const fileList = ['file1.txt','file2.txt','file3.txt'];
*
* // asynchronous function that returns the file size, transformed to human-readable format
* // e.g. 1024 bytes = 1KB, 1234 bytes = 1.21 KB, 1048576 bytes = 1MB, etc.
* function transformFileSize(acc, value, key, callback) {
* fs.stat(value, function(err, stat) {
* if (err) {
* return callback(err);
* }
* acc[key] = formatBytes(stat.size);
* callback(null);
* });
* }
*
* // Using callbacks
* async.transform(fileList, transformFileSize, function(err, result) {
* if(err) {
* console.log(err);
* } else {
* console.log(result);
* // [ '1000 Bytes', '1.95 KB', '2.93 KB' ]
* }
* });
*
* // Using Promises
* async.transform(fileList, transformFileSize)
* .then(result => {
* console.log(result);
* // [ '1000 Bytes', '1.95 KB', '2.93 KB' ]
* }).catch(err => {
* console.log(err);
* });
*
* // Using async/await
* (async () => {
* try {
* let result = await async.transform(fileList, transformFileSize);
* console.log(result);
* // [ '1000 Bytes', '1.95 KB', '2.93 KB' ]
* }
* catch (err) {
* console.log(err);
* }
* })();
*
* @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
*
* // helper function that returns human-readable size format from bytes
* function formatBytes(bytes, decimals = 2) {
* // implementation not included for brevity
* return humanReadbleFilesize;
* }
*
* const fileMap = { f1: 'file1.txt', f2: 'file2.txt', f3: 'file3.txt' };
*
* // asynchronous function that returns the file size, transformed to human-readable format
* // e.g. 1024 bytes = 1KB, 1234 bytes = 1.21 KB, 1048576 bytes = 1MB, etc.
* function transformFileSize(acc, value, key, callback) {
* fs.stat(value, function(err, stat) {
* if (err) {
* return callback(err);
* }
* acc[key] = formatBytes(stat.size);
* callback(null);
* });
* }
*
* // Using callbacks
* async.transform(fileMap, transformFileSize, function(err, result) {
* if(err) {
* console.log(err);
* } else {
* console.log(result);
* // { f1: '1000 Bytes', f2: '1.95 KB', f3: '2.93 KB' }
* }
* });
*
* // Using Promises
* async.transform(fileMap, transformFileSize)
* .then(result => {
* console.log(result);
* // { f1: '1000 Bytes', f2: '1.95 KB', f3: '2.93 KB' }
* }).catch(err => {
* console.log(err);
* });
*
* // Using async/await
* async () => {
* try {
* let result = await async.transform(fileMap, transformFileSize);
* console.log(result);
* // { f1: '1000 Bytes', f2: '1.95 KB', f3: '2.93 KB' }
* }
* catch (err) {
* console.log(err);
* }
* }
*
*/
export default function transform (coll, accumulator, iteratee, callback) {
if (arguments.length <= 3 && typeof accumulator === 'function') {
callback = iteratee;
iteratee = accumulator;
accumulator = Array.isArray(coll) ? [] : {};
}
callback = once(callback || promiseCallback());
var _iteratee = wrapAsync(iteratee);
eachOf(coll, (v, k, cb) => {
_iteratee(accumulator, v, k, cb);
}, err => callback(err, accumulator));
return callback[PROMISE_SYMBOL]
}
</code></pre>
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<h3>Methods:</h3><ul class="nav methods"><li class="toc-header"><a href="docs.html#collections">Collections</a></li><li data-type="method" class="toc-method"><a href="docs.html#concat">concat</a></li><li data-type="method" class="toc-method"><a href="docs.html#concatLimit">concatLimit</a></li><li data-type="method" class="toc-method"><a href="docs.html#concatSeries">concatSeries</a></li><li data-type="method" class="toc-method"><a href="docs.html#detect">detect</a></li><li data-type="method" class="toc-method"><a href="docs.html#detectLimit">detectLimit</a></li><li data-type="method" class="toc-method"><a href="docs.html#detectSeries">detectSeries</a></li><li data-type="method" class="toc-method"><a href="docs.html#each">each</a></li><li data-type="method" class="toc-method"><a href="docs.html#eachLimit">eachLimit</a></li><li data-type="method" class="toc-method"><a href="docs.html#eachOf">eachOf</a></li><li data-type="method" class="toc-method"><a href="docs.html#eachOfLimit">eachOfLimit</a></li><li data-type="method" class="toc-method"><a href="docs.html#eachOfSeries">eachOfSeries</a></li><li data-type="method" class="toc-method"><a href="docs.html#eachSeries">eachSeries</a></li><li data-type="method" class="toc-method"><a href="docs.html#every">every</a></li><li data-type="method" class="toc-method"><a href="docs.html#everyLimit">everyLimit</a></li><li data-type="method" class="toc-method"><a href="docs.html#everySeries">everySeries</a></li><li data-type="method" class="toc-method"><a href="docs.html#filter">filter</a></li><li data-type="method" class="toc-method"><a href="docs.html#filterLimit">filterLimit</a></li><li data-type="method" class="toc-method"><a href="docs.html#filterSeries">filterSeries</a></li><li data-type="method" class="toc-method"><a href="docs.html#groupBy">groupBy</a></li><li data-type="method" class="toc-method"><a href="docs.html#groupByLimit">groupByLimit</a></li><li data-type="method" 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