Lebab

Lebab transpiles your ES5 code to ES6/ES7. It does exactly the opposite of what Babel does. If you want to understand what Lebab exactly does, try the live demo.

Install

Install it using npm:

$ npm install -g lebab

Usage

Convert your old-fashioned code using the lebab cli tool, enabling a specific transformation:

$ lebab es5.js -o es6.js --transform let

Or transform an entire directory of files in-place:

# .js files only
$ lebab --replace src/js/ --transform arrow
# For other file extensions, use explicit globbing
$ lebab --replace 'src/js/**/*.jsx' --transform arrow

For all the possible values for --transform option see the detailed docs below or use --help from command line.

Features and known limitations

The recommended way of using Lebab is to apply one transform at a time, read what exactly the transform does and what are its limitations, apply it for your code and inspect the diff carefully.

Safe transforms

These transforms can be applied with relatively high confidence. They use pretty straight-forward and strict rules for changing the code. The resulting code should be almost 100% equivalent of the original code.

• arrow - callbacks to arrow functions
  • Converts bound functions like function(){}.bind(this)
  • not applied to unbound functions that use this
  • not applied to functions that use arguments
  • not applied to object properties (use obj-method transform)
  • does not remove that = this assignments
  • LIMITATION: can mess up prototype-based classes, run the class transform first to prevent this.
• arrow-return - drop return statements in arrow functions
  • converts immediate return { return x; } to => x
  • applies to arrow functions and nested arrow functions
  • LIMITATION only applies to arrow functions (run the arrow transform first)
• for-of - for loop to for-of loop
  • uses name item for loop variable when loop body begins with var item = array[i];
  • does not work when no such alias defined at the start of loop body
  • LIMITATION requires let/const variables (run the let transform first)
• for-each - for loop to Array.forEach()
  • uses name item for forEach parameter when loop body begins with var item = array[i];
  • does not work when no such alias defined at the start of loop body
  • adds index parameter when loop body makes use of the index variable.
  • LIMITATION requires let/const variables (run the let transform first)
• arg-rest - use of arguments to function(...args)
  • does not perform the transform when args variable already exists
  • always names the rest-parameter to args
  • LIMITATION does not transform functions with formal parameters
  • LIMITATION does not remove uses of Array.slice.call(arguments)
• arg-spread - use of apply() to spread operator
  • recognizes obj.method.apply(obj, args)
  • recognizes func.apply(undefined, args)
• obj-method - function values in object to methods
  • LIMITATION does not convert named function expressions
  • does not convert arrow-functions
• obj-shorthand - {foo: foo} to {foo}
  • ignores numeric and NaN properties
  • does not convert string properties
• no-strict - removal of "use strict" directives
  • does not touch stuff like x = "use strict";
• exponent - Math.pow() to ** operator (ES7)
  • Full support for all new syntax from ES7
• multi-var - single var x,y; declaration to multiple var x; var y; (refactor)
  • Not related to any new syntax feature
  • EXPERIMENT to see if Lebab could be a more generic refactoring helper

Unsafe transforms

These transforms should be applied with caution. They either use heuristics which can't guarantee that the resulting code is equivalent of the original code, or they have significant bugs which can result in breaking your code.

• let - var to let/const
  • never modified variables are converted to const
  • properly recognizes block-scoping
  • splits single var declaration to multiple let/const declarations if needed
  • recognizes vars defined/assigned using destructuring
  • vars that conflict with block-scoping are not converted
  • repeated declarations of the same var are not converted
  • existing let/const are not converted
  • BUG fails with repeated variable definitions that use destructuring
  • BUG fails with closure over a loop variable
  • BUG fails when function closes over variable declared after function is called
• class - function/prototypes to classes
  • recognizes Foo.prototype.method = function(){ ... };
  • recognizes Foo.prototype = { ...methods... };
  • recognizes static methods like Foo.method = function(){ ... };
  • recognizes getters/setters defined with Object.defineProperty()
  • recognizes inheritance with Child.prototype = new Parent()
  • recognizes inheritance with util.inherits(Child, Parent);
  • converts superclass constructor calls to super()
  • converts superclass method calls to super.method()
  • LIMITATION does not require super() call in subclass constructor
  • LIMITATION does not enforce super() call position in subclass constructor
  • LIMITATION does not support namespaced classes
• commonjs - CommonJS module definition to ES6 modules
  • converts var foo = require("foo") to import foo from "foo"
  • converts var bar = require("foo").bar to import {bar} from "foo"
  • converts var {bar} = require("foo") to import {bar} from "foo"
  • converts module.exports = <anything> to export default <anything>
  • converts exports.foo = function(){} to export function foo(){}
  • converts exports.Foo = class {} to export class Foo {}
  • converts exports.foo = 123 to export var foo = 123
  • converts exports.foo = bar to export {bar as foo}
  • LIMITATION does not check if named export conflicts with existing variable names
  • LIMITATION Ignores imports/exports inside nested blocks/functions
  • LIMITATION only handles require() calls in var declarations
  • LIMITATION does not ensure that imported variable is treated as const
  • LIMITATION does not ensure named exports are imported with correct ES6 syntax
• template - string concatenation to template strings
  • converts variables and arbitrary expressions to ${...}
  • BUG removes indentation of multi-line strings
  • LIMITATION ignores difference between .toString() and .valueOf()
• default-param - default parameters instead of a = a || 2
  • recognizes a = a || 2
  • recognizes a = a ? a : 2
  • recognizes a = a === undefined ? 2 : a
  • recognizes a = typeof a === 'undefined' ? 2 : a
  • LIMITATION transforming a = a || 2 does not produce strictly equivalent code
• destruct-param - use destructuring for objects in function parameters
  • converts (obj) => obj.a + obj.b to ({a, b}) => a + b
  • does not transform when conflicts with existing variables
  • does not transform when object properties are modified
  • LIMITATION Only objects with maximum of 4 properties are transformed
  • BUG Can conflict with variables introduced by the transform itself
• includes - array.indexOf(foo) !== -1 to array.includes(foo) (ES7)
  • works for both strings and arrays
  • converts !== -1 to array.includes(foo)
  • converts === -1 to !array.includes(foo)
  • recognizes all kinds of comparisons >= 0, > -1, etc
  • recognizes both indexOf() != -1 and -1 != indexOf()
  • LIMITATION does not detect that indexOf() is called on an actual Array or String.

Programming API

Simply import and call the transform() function:

import {transform} from 'lebab';
const {code, warnings} = transform(
  'var f = function(a) { return a; };', // code to transform
  ['let', 'arrow', 'arrow-return'] // transforms to apply
);
console.log(code); // -> "const f = a => a;"

The warnings will be an array of objects like:

[
  {line: 12, msg: 'Unable to transform var', type: 'let'},
  {line: 45, msg: 'Can not use arguments in arrow function', type: 'arrow'},
]

Most of the time there won't be any warnings and the array will be empty.

Editor plugins

Alternatively one can use Lebab through plugins in the following editors:

• Atom: atom-lebab
• Sublime: lebab-sublime or Lebab ES6 Transform
• VSCode: vscode-lebab

What's next?

Which feature should Lebab implement next? Let us know by creating an issue or voicing your opinion in existing one.

Want to contribute? Read how Lebab looks for patterns in syntax trees.