vjeux / es5-shim

ECMAScript 5 compatibility shims for legacy JavaScript engines

es5-shim.js and es5-shim.min.js monkey-patch a JavaScript context to contain all EcmaScript 5 methods that can be faithfully emulated with a legacy JavaScript engine.

es5-sham.js and es5-sham.min.js monkey-patch other ES5 methods as closely as possible. For these methods, as closely as possible to ES5 is not very close. Many of these shams are intended only to allow code to be written to ES5 without causing run-time errors in older engines. In many cases, this means that these shams cause many ES5 methods to silently fail. Decide carefully whether this is what you want.

Tests

The tests are written with the Jasmine BDD test framework. To run the tests, navigate to /tests/.

In order to run against the shim-code, the tests attempt to kill the current implementation of the missing methods. This happens in /tests/helpers/h-kill.js. So in order to run the tests against the built-in methods, invalidate that file somehow (comment-out, delete the file, delete the script-tag, etc.).

Shims

Complete tests

• Array.prototype.every
• Array.prototype.filter
• Array.prototype.forEach
• Array.prototype.indexOf
• Array.prototype.lastIndexOf
• Array.prototype.map
• Array.prototype.some
• Array.prototype.reduce
• Array.prototype.reduceRight
• Array.isArray
• Date.now
• Date.prototype.toJSON
• Function.prototype.bind
  • ⚠️ Caveat: the bound function's length is always 0.
  • ⚠️ Caveat: the bound function has a prototype property.
  • ⚠️ Caveat: bound functions do not try too hard to keep you from manipulating their arguments and caller properties.
  • ⚠️ Caveat: bound functions don't have checks in call and apply to avoid executing as a constructor.
• Number.prototype.toFixed
• Object.keys
• String.prototype.split
• String.prototype.trim
• Date.parse (for ISO parsing)
• Date.prototype.toISOString

Shams

• ⚠️ Object.create

  For the case of simply "begetting" an object that inherits prototypically from another, this should work fine across legacy engines.

  ⚠️ Object.create(null) will work only in browsers that support prototype assignment. This creates an object that does not have any properties inherited from Object.prototype. It will silently fail otherwise.

  ⚠️ The second argument is passed to Object.defineProperties which will probably fail either silently or with extreme predudice.

• ⚠️ Object.getPrototypeOf

  This will return "undefined" in some cases. It uses __proto__ if it's available. Failing that, it uses constructor.prototype, which depends on the constructor property of the object's prototype having not been replaced. If your object was created like this, it won't work:

    function Foo() {
    }
    Foo.prototype = {};
  

  Because the prototype reassignment destroys the constructor property.

  This will work for all objects that were created using Object.create implemented with this library.

• ⚠️ Object.getOwnPropertyNames

  This method uses Object.keys, so it will not be accurate on legacy engines.

• Object.isSealed

  Returns "false" in all legacy engines for all objects, which is conveniently guaranteed to be accurate.

• Object.isFrozen

  Returns "false" in all legacy engines for all objects, which is conveniently guaranteed to be accurate.

• Object.isExtensible

  Works like a charm, by trying very hard to extend the object then redacting the extension.

May fail

• ⚠️ Object.getOwnPropertyDescriptor

  The behavior of this shim does not conform to ES5. It should probably not be used at this time, until its behavior has been reviewed and been confirmed to be useful in legacy engines.

• ⚠️ Object.defineProperty

  In the worst of circumstances, IE 8 provides a version of this method that only works on DOM objects. This sham will not be installed. The given version of defineProperty will throw an exception if used on non-DOM objects.

  In slightly better circumstances, this method will silently fail to set "writable", "enumerable", and "configurable" properties.

  Providing a getter or setter with "get" or "set" on a descriptor will silently fail on engines that lack "defineGetter" and "defineSetter", which include all versions of IE.

  https://github.com/kriskowal/es5-shim/issues#issue/5

• ⚠️ Object.defineProperties

  This uses the Object.defineProperty shim

• Object.seal

  Silently fails on all legacy engines. This should be fine unless you are depending on the safety and security provisions of this method, which you cannot possibly obtain in legacy engines.

• Object.freeze

  Silently fails on all legacy engines. This should be fine unless you are depending on the safety and security provisions of this method, which you cannot possibly obtain in legacy engines.

• Object.preventExtensions

  Silently fails on all legacy engines. This should be fine unless you are depending on the safety and security provisions of this method, which you cannot possibly obtain in legacy engines.