Static scope analysis and transpilation of ES6 block scoped const and let variables, to ES3.

  • defs

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defs is done. I recommend migrating to the TypeScript tsc compiler because it does what defs does as good or better, and it does much more.
Static scope analysis and transpilation of ES6 block scoped const and let variables, to ES3.
Node already supports const and let so you can use that today (run node --harmony and "use strict"). defs.js enables you to do the same for browser code. While developing you can rely on the experimental support in Chrome (chrome://flags, check Enable experimental JavaScript). defs.js is also a pretty decent static scope analyzer/linter.
The talk LET's CONST together, right now (with ES3) from Front-Trends 2013 (slides) includes more information about let, const and defs.js. See also the blog post ES3 <3 block scoped const and let => defs.js.

Installation and usage

npm install -g defs
Then run it as defs file.js. The errors (if any) will go to stderr, the transpiled source to stdout, so redirect it like defs file.js > output.js. More command line options are coming.
There's also a Grunt plugin, see grunt-defs.
See for a description of the self-build and the browser bundle.


MIT, see LICENSE file.




defs looks for a defs-config.json configuration file in your current directory. If not found there, it searches parent directories until it hits /. You may instead pass a custom defs-config.json using --config, i.e. defs --config path/to/defs-config.json file.js > output.js.
Example defs-config.json:
    "environments": ["node", "browser"],

    "globals": {
        "my": false,
        "hat": true
    "loopClosures": "iife",
    "disallowVars": false,
    "disallowDuplicated": true,
    "disallowUnknownReferences": true
globals lets you list your program's globals, and indicate whether they are writable (true) or read-only (false), just like jshint.
environments lets you import a set of pre-defined globals, here node and browser. These default environments are borrowed from jshint (see jshintglobals/vars.js).
loopClosures (defaults to false) can be set to "iife" to enable transformation of loop-closures via immediately-invoked function expressions.
disallowVars (defaults to false) can be enabled to make usage of var an error.
disallowDuplicated (defaults to true) errors on duplicated var definitions in the same function scope.
disallowUnknownReferences (defaults to true) errors on references to unknown global variables.
ast (defaults to false) produces an AST instead of source code (experimental).
stats (defaults to false) prints const/let statistics and renames (experimental).
parse (defaults to null) lets you provide a custom parse function if you use defs as an API. By default it will use require("esprima").parse.


Input example.js:
"use strict";
function fn() {
    const y = 0;
    for (let x = 0; x < 10; x++) {
        const y = x * 2;
        const z = y;
    console.log(y); // prints 0

Output from running defs example.js:
"use strict";
function fn() {
    var y = 0;
    for (var x = 0; x < 10; x++) {
        var y$0 = x * 2;
        var z = y$0;
    console.log(y); // prints 0

defs.js used as a library

npm install defs, then:
const defs = require("defs");
const options = {};
const src = "const x = 1";
const res = defs(src, options);
assert(res.src === "var x = 1");

// you can also pass an AST (with loc and range) instead of a string to defs
const ast = require("esprima").parse(src, {loc: true, range: true});
const res = defs(ast, {ast: true}); // AST-in, AST-out
// inspect res.ast

res object:
    src: string // on success
    errors: array of error messages // on errors
    stats: statistics object (toStringable)
    ast: transformed ast // when options.ast is set


defs.js strives to transpile your program as true to ES6 block scope semantics as possible while being as maximally non-intrusive as possible.
It can optionally transform loop closures via IIFE's (when possible), if you include "loopClosures": "iife" in your defs-config.json. More info in
See for other minor differences.