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This plugins helps with automatic #__PURE__ annotation insertion. It add the comment to top level call expressions and new expressions in assignment contexts (those are considered by the plugin as side effect free). This helps UglifyJS to perform dead code elimination more efficiently and therefore reduces the bundle sizes for the consumers.

NOTE: It might break your code, so the caution is advised. Target audience for the plugin are libraries, which in vast major of use cases do not introduce side effects in top level calls. That doesn't mean that application bundles cannot benefit from the plugin.

Pure calls

// pure call
var inc = add(1)
// clearly impure - no assignment context
mutate({ prop: 'value' })

Top level calls

Top level call (in terms of this plugin) is one that gets executed during script initialization. So it is every call located at the root of a file, but also a call in an IIFE that gets executed at startup (including nested ones).

var a = topLevelCall()
b = function () {
topLevelIIFEs = (function () {
  var c = (function () {
    var d = (function () {
      var e = topLevelCall()


npm install --save-dev babel-plugin-annotate-pure-calls


Via .babelrc (Recommended)


  "plugins": ["annotate-pure-calls"]


babel --plugins annotate-pure-calls script.js

Via Node API

require("babel-core").transform("var inc = add(1)", {
  plugins: ["annotate-pure-calls"]

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