• babel-plugin-prototype-prop-define
  • background

babel-plugin-prototype-prop-define

A babel plugin for working around javascript's "override mistake" when dealing with frozen Primordials (built-ins).

transformation

If your primordials are frozen, such as in SES, assigning keys that are found on the prototype will throw an error.

Object.freeze(Object.prototype)

const x = {}
x.toString = () => 'hello'
// => TypeError: Cannot assign to read only property 'toString' of object '#<Object>'

Since this is a common, this plugin is provided to transform code into a safe form using Object.defineProperty:

Object.freeze(Object.prototype)

const x = {}
(function (parent, key, value) {
  Object.defineProperty(parent, key, {
    value: value,
    writable: true,
    enumerable: true,
    configurable: true
  })
  return value;
})(x, "toString", () => 'hello');
x.toString()
// => 'hello'

The code is transformed into an IIFE in order to ensure the "parent" (x), "key" (y) and "value" (() => 'hello') statements are evaluated exactly once, and that the whole expression resolves to the value of the "value" statement. This is needed because Object.defineProperty calls resolve to undefined.

For "computed" assignment, the resulting code is a little uglier.

const x = {};
const y = 'toString';

x[y] = () => 'hello'

becomes

const x = {};
const y = 'toString';

(function (parent, key, value) {
  ["constructor", "__defineGetter__", "__defineSetter__", "hasOwnProperty", "__lookupGetter__", "__lookupSetter__", "isPrototypeOf", "propertyIsEnumerable", "toString", "valueOf", "__proto__", "toLocaleString", "length", "concat", "find", "findIndex", "pop", "push", "shift", "unshift", "slice", "splice", "includes", "indexOf", "keys", "entries", "forEach", "filter", "map", "every", "some", "reduce", "reduceRight", "join", "reverse", "sort", "lastIndexOf", "copyWithin", "fill", "values", "name", "arguments", "caller", "apply", "bind", "call", "message"].includes(key) ? Object.defineProperty(parent, key, {
    value: value,
    writable: true,
    enumerable: true,
    configurable: true
  }) : parent[key] = value;
  return value;
})(x, y, () => true);

Here the check that is normally performed at parse time (checking if the key is in this whitelist), is performed at runtime to determine if normal assignment or Object.defineProperty should be used.