defs

defs.js

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.

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 is coming.

Configuration

defs looks for a defs-config.json configuration file in your current directory. It will search for it in parent directories soon as you'd expect.

Example defs-config.json:

{
    "environments": ["node", "browser"],

    "globals": {
        "my": false,
        "hat": true
    },
    "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 jshint_globals/vars.js).

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.

Example

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
}
fn();

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
}
fn();

License

MIT, see LICENSE file.