foreach之前的Javascript分配（我不知道这叫什么）

Question

I'm going through some example code for looping through directories in Chrome App API and I've come across a statement I haven't seen before and don't know what to search.

function recurseDirectory(dirEntry, callback) {
    var dirsLeft = 1;
    var rootEntries;
    let helper = (dirEntry, isRoot) => {
        dirEntry.createReader().readEntries(entries => {
            dirsLeft--;

            if (isRoot) {
                rootEntries = entries;
            }
            //This line is what I'm confused about
            (dirEntry.entries = entries).forEach(entry => {
                if (entry.isDirectory) {
                    dirsLeft++;
                    helper(entry);
                }
            });

            if (!dirsLeft) {
                callback(rootEntries);
            }
        });
    }

    helper(dirEntry, true);
}

I can't run this code yet since it's just a snippet so I'm still dissecting it.

The part that I don't know is (dirEntry.entries = entries).forEach {...} . I tried looking up assignment foreach , equal foreach but no results for this.

What is this operation/syntax called and what does it do?

Answer 1

It's just an ordinary variable assignment - however, placing it in brackets means that you're assigning dirEntry.entries the value of entries , and iterating through entries with the same statement - it's just shorthand for:

dirEntry.entries = entries;
entries.forEach(entry => {...});

Answer 2

This is not something else but an Assignment Expression. (dirEntry.entries = entries) will return the value of entries on which forEach will be applied and in also dirEntry.entries to entires . Its doing to things in one line.

• Changing the value of data.entires to entires
• return entires on which forEach is applied.

Answer 3

它只是数组的赋值和forEach的立即执行，现在由entries和dirEntry.entries引用该数组。

Answer 4

This is just a lazy guy trying to save himself couple key press. It's equiv to

dirEntry.entries = entries;
entries.forEach(...)