function concat(coll, iteratee, callback) {
        return concatLimit$1(coll, Infinity, iteratee, callback)
    }
Applies `iteratee` to each item in `coll`, concatenating the results. Returns
the concatenated list. The `iteratee`s are called in parallel, and the
results are concatenated as they return. The results array will be returned in
the original order of `coll` passed to the `iteratee` function.

@name concat
@static
@memberOf module:Collections
@method
@category Collection
@alias flatMap
@param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
@param {AsyncFunction} iteratee - A function to apply to each item in `coll`,
which should use an array as its result. Invoked with (item, callback).
@param {Function} [callback] - A callback which is called after all the
`iteratee` functions have finished, or an error occurs. Results is an array
containing the concatenated results of the `iteratee` function. Invoked with
(err, results).
@returns A Promise, if no callback is passed
@example

// dir1 is a directory that contains file1.txt, file2.txt
// dir2 is a directory that contains file3.txt, file4.txt
// dir3 is a directory that contains file5.txt
// dir4 does not exist

let directoryList = ['dir1','dir2','dir3'];
let withMissingDirectoryList = ['dir1','dir2','dir3', 'dir4'];

// Using callbacks
async.concat(directoryList, fs.readdir, function(err, results) {
   if (err) {
       console.log(err);
   } else {
       console.log(results);
       // [ 'file1.txt', 'file2.txt', 'file3.txt', 'file4.txt', file5.txt ]
   }
});

// Error Handling
async.concat(withMissingDirectoryList, fs.readdir, function(err, results) {
   if (err) {
       console.log(err);
       // [ Error: ENOENT: no such file or directory ]
       // since dir4 does not exist
   } else {
       console.log(results);
   }
});

// Using Promises
async.concat(directoryList, fs.readdir)
.then(results => {
    console.log(results);
    // [ 'file1.txt', 'file2.txt', 'file3.txt', 'file4.txt', file5.txt ]
}).catch(err => {
     console.log(err);
});

// Error Handling
async.concat(withMissingDirectoryList, fs.readdir)
.then(results => {
    console.log(results);
}).catch(err => {
    console.log(err);
    // [ Error: ENOENT: no such file or directory ]
    // since dir4 does not exist
});

// Using async/await
async () => {
    try {
        let results = await async.concat(directoryList, fs.readdir);
        console.log(results);
        // [ 'file1.txt', 'file2.txt', 'file3.txt', 'file4.txt', file5.txt ]
    } catch (err) {
        console.log(err);
    }
}

// Error Handling
async () => {
    try {
        let results = await async.concat(withMissingDirectoryList, fs.readdir);
        console.log(results);
    } catch (err) {
        console.log(err);
        // [ Error: ENOENT: no such file or directory ]
        // since dir4 does not exist
    }
}