Backs out file tree changes
Last updated 9 months ago by stefanpenner .
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FSTree provides the means to calculate a patch (set of operations) between one file system tree and another.

The possible operations are:

  • unlink – remove the specified file
  • rmdir – remove the specified folder
  • mkdir – create the specified folder
  • create – create the specified file
  • change – update the specified file to reflect changes

The operations chosen aim to minimize the amount of IO required to apply a given patch. For example, a naive rm -rf of a directory tree is actually quite costly, as child directories must be recursively traversed, entries stated.. etc, all to figure out what first must be deleted. Since we patch from tree to tree, discovering new files is both wasteful and un-needed.

The operations will also be provided in a correct order, allowing us to safely replay operations without having to first confirm the FS is as we expect. For example, unlinks for files will occur before a rmdir of those files' parent dir. Although the ordering will be safe, a specific order is not guaranteed.

A simple example:

const FSTree = require('fs-tree-diff');
const current = FSTree.fromPaths([

const next = FSTree.fromPaths([

current.calculatePatch(next) === [
  ['unlink', 'a.js', entryA],
  ['create', 'b.js', entryB]

A slightly more complicated example:

const FSTree = require('fs-tree-diff');
const current = FSTree.fromPaths([

const next = FSTree.fromPaths([

current.calculatePatch(next) === [
  ['unlink', 'a.js', entryA],
  ['create', 'b.js', entryB],
  ['mkdir', 'b/c', entryBC],
  ['create', 'b/c/d.js', entryBCD],
  ['create', 'b/e.js', entryBE]
  ['unlink', 'b/f.js', entryBF],

Now, the above examples do not demonstrate change operations. This is because when providing only paths, we do not have sufficient information to check if one entry is merely different from another with the same relativePath.

For this, FSTree supports more complex input structure. To demonstrate, we will use the walk-sync module, which provides higher fidelity input, allowing FSTree to also detect changes. (See also the documentation for walkSync.entries.)

const walkSync = require('walk-sync');

// path/to/root/foo.js
// path/to/root/bar.js
const current = new FSTree({
  entries: walkSync.entries('path/to/root')

writeFileSync('path/to/root/foo.js', 'new content');
writeFileSync('path/to/root/baz.js', 'new file');

const next = new FSTree({
  entries: walkSync.entries('path/to/root')

current.calculatePatch(next) === [
  ['change', 'foo.js', entryFoo], // mtime + size changed, so this input is stale and needs updating.
  ['create', 'baz.js', entryBaz]  // new file, so we should create it
  /* bar stays the same and is left inert*/

The entry objects provided depend on the operation. For rmdir and unlink operations, the current entry is provided. For mkdir, change and create operations the new entry is provided.


The public API is:

  • FSTree.fromPaths initialize a tree from an array of string paths.

  • FSTree.fromEntries initialize a tree from an array of Entry objects. Each entry must have the following properties (but may have more):

    • relativePath
    • mode
    • size
    • mtime
  • FSTree.applyPatch(inputDir, outputDir, patch, delegate) applies the given patch from the input directory to the output directory. You can optionally provide a delegate object to handle individual types of patch operations.

  • FSTree.prototype.calculatePatch(newTree, isEqual) calculate a patch against newTree. Optionally specify a custom isEqual (see Change Calculation).

  • FSTree.prototype.calculateAndApplyPatch(newTree, inputDir, outputDir, delegate) does a calculatePatch followed by applyPatch.

  • FSTree.prototype.addEntries(entries, options) adds entries to an existing tree. Options are the same as for FSTree.fromEntries. Entries added with the same path will overwrite any existing entries.

  • FSTree.prototype.addPaths(paths, options) adds paths to an existing tree. Options are the same as for FSTree.fromPaths. If entries already exist for any of the paths added, those entries will be updated.

  • Entry.fromStat(relativePath, stat) creates an Entry from a given path and fs.Stats object. It can then be used with fromEntries or addEntries.

The trees returned from fromPaths and fromEntries are relative to some base directory. calculatePatch, applyPatch and calculateAndApplyPatch all assume that the base directory has not changed.


FSTree.fromPaths, FSTree.fromEntries, FSTree.prototype.addPaths, and FSTree.prototype.addEntries all validate their inputs. Inputs must be sorted, path-unique (i.e. two entries with the same relativePath but different sizes would still be illegal input) and include intermediate directories.

For example, the following input is invalid

  // => missing a/ and a/b/

To have FSTree sort and expand (include intermediate directories) for you, add the option sortAndExpand).

], { sortAndExpand: true });

// The above is equivalent to



FSTree.fromEntries requires you to supply your own Entry objects. Your entry objects must contain the following properties:

  • relativePath
  • mode
  • size
  • mtime

They must also implement the following API:

  • isDirectory() true iff this entry is a directory

FSTree.fromEntries composes well with the output of walkSync.entries:

const walkSync = require('walk-sync');

// path/to/root/foo.js
// path/to/root/bar.js
const current = FSTree.fromEntries(walkSync.entries('path/to/root'));

Change Calculation

When a prior entry has a relativePath that matches that of a current entry, a change operation is included if the new entry is different from the previous entry. This is determined by calling isEqual, the optional second argument to calculatePatch. If no isEqual is provided, a default isEqual is used.

The default isEqual treats directories as always equal and files as different if any of the following properties have changed.

  • mode
  • size
  • mtime

User specified isEqual will often want to use the default isEqual, so it is exported on FSTree.


const defaultIsEqual = FSTree.defaultIsEqual;

function isEqualCheckingMeta(a, b) {
  return defaultIsEqual(a, b) && isMetaEqual(a, b);

function isMetaEqual(a, b) {
  // ...

Patch Application

When you want to apply changes from one tree to another easily, you can use the FSTree.applyPatch method. For example, given:

const patch = oldInputTree.calculatePatch(newInputTree);
const inputDir = 'src';
const outputDir = 'dist';
FSTree.applyPatch(inputDir, outputDir, patch);

It will apply the patch changes to dist while using src as a reference for non-destructive operations (mkdir, create, change). If you want to calculate and apply a patch without any intermediate operations, you can do:

const inputDir = 'src';
const outputDir = 'dist';
oldInputTree.calculateAndApplyPatch(newInputTree, inputDir, outputDir);

You can optionally provide a delegate object to handle applying specific types of operations:

let createCount = 0;
FSTree.applyPatch(inputDir, outputDir, patch, {
  create: function(inputPath, outputPath, relativePath) {
    copy(inputPath, outputPath);

The available delegate functions are the same as the supported operations: unlink, rmdir, mkdir, create, and change. Each delegate function receives the reference inputPath, the outputPath, and relativePath of the file or directory for which to apply the operation.

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