node package manager



Detect copy-pasted and structurally similar JavaScript code. Requires Node.js 6.0+, and supports ES6, JSX as well as Flow. Note: the project has been mostly rewritten for the 0.10 release and saw several breaking changes.

Build Status


We've all had to deal with code smell, and duplicate code is a common source. While some instances are easy to spot, this type of searching is the perfect use-case for a helpful CLI tool.

Existing solutions do exist for this purpose, but some struggle with code that has wildly varying identifiers or literals, and others have lackluster support for the JS ecosystem: ES6, JSX, Flow, ignoring module declarations and imports, etc.

And copy-pasted code is but one type of code duplication. Common boilerplate and repeated logic can be identified as well using jsinspect, since it doesn't operate directly on tokens - it uses the ASTs of the parsed code.

You have the freedom to specify a threshold determining the smallest subset of nodes to analyze. This will identify code with a similar structure, based on the AST node types, e.g. BlockStatement, VariableDeclaration, ObjectExpression, etc. By default, it searches nodes with matching identifiers and literals for copy-paste oriented detection, but this can be disabled. For context, identifiers include the names of variables, methods, properties, etc, while literals are strings, numbers, etc.

The tool accepts a list of paths to parse and prints any found matches. Any directories among the paths are walked recursively, and only .js and .jsx files are analyzed. You can explicitly pass file paths that include a different extension as well. Any node_modules and bower_components dirs are also ignored.



It can be installed via npm using:

npm install -g jsinspect


Usage: jsinspect [options] <paths ...>

Detect copy-pasted and structurally similar JavaScript code
Example use: jsinspect -I -L -t 20 --ignore "test" ./path/to/src


  -h, --help                         output usage information
  -V, --version                      output the version number
  -t, --threshold <number>           number of nodes (default: 30)
  -m, --min-instances <number>       min instances for a match (default: 2)
  -c, --config [config]              path to config file (default: .jsinspectrc)
  -r, --reporter [default|json|pmd]  specify the reporter to use
  -I, --no-identifiers               do not match identifiers
  -L, --no-literals                  do not match literals
  -C, --no-color                     disable colors
  --ignore <pattern>                 ignore paths matching a regex
  --truncate <number>                length to truncate lines (default: 100, off: 0)
  --debug                            print debug information

If a .jsinspectrc file is located in the project directory, its values will be used in place of the defaults listed above. For example:

  "threshold":     30,
  "identifiers":   true,
  "literals":      true,
  "color":         true,
  "minInstances":  2,
  "ignore":        "test|spec|mock",
  "reporter":      "json",
  "truncate":      100,

On first use with a project, you may want to run the tool with the following options, while running explicitly on the lib/src directories, and not the test/spec dir.

jsinspect -t 50 --ignore "test" ./path/to/src

From there, feel free to try decreasing the threshold, ignoring identifiers using the -I flag and ignoring literals with -L. A lower threshold may lead you to discover new areas of interest for refactoring or cleanup.


It's simple to run jsinspect on your library source as part of a build process. It will exit with an error code of 0 when no matches are found, resulting in a passing step, and a positive error code corresponding to its failure. For example, with Travis CI, you could add the following entries to your .travis.yml:

  - "npm install -g jsinspect"
  - "jsinspect ./path/to/src"

Note that in the above example, we're using a threshold of 30 for detecting structurally similar code. A higher threshold may be appropriate as well.

To have jsinspect run with each job, but not block or fail the build, you can use something like the following:

  - "jsinspect ./path/to/src || true"


Aside from the default reporter, both JSON and PMD CPD-style XML reporters are available. Note that in the JSON example below, indentation and formatting has been applied. Furthermore, the id property available in these reporters is useful for parsing by automatic scripts to determine whether or not duplicate code has changed between builds.


      "code":"function intersectionA(array1, array2) {\n  array1.filter(function(n) {\n    return array2.indexOf(n) != -1;\n  });\n}"
      "code":"function intersectionB(arrayA, arrayB) {\n  arrayA.filter(function(n) {\n    return arrayB.indexOf(n) != -1;\n  });\n}"


<?xml version="1.0" encoding="utf-8"?>
<duplication lines="10" id="6ceb36d5891732db3835c4954d48d1b90368a475">
<file path="/jsinspect/spec/fixtures/intersection.js" line="1"/>
<file path="/jsinspect/spec/fixtures/intersection.js" line="7"/>
function intersectionA(array1, array2) {
  array1.filter(function(n) {
    return array2.indexOf(n) != -1;
function intersectionB(arrayA, arrayB) {
  arrayA.filter(function(n) {
    return arrayB.indexOf(n) != -1;