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WhatsMissing

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Kevin Reid edited this page Apr 16, 2015 · 1 revision

(legacy summary: What JavaScript constructs are proposed for EcmaScript or commonly used, and absent from Valija?) (legacy labels: Phase-Design)

Introduction

Valija is approximately a subset of the ES5 strict mode. But existing libraries are built for "consensus JavaScript", also known as "ES3 + Reality" (hereafter "ES3R"), as represented by what browsers currently do. On this page we try to enumerate all features of ES3R not available in Valija, so that we can more quickly spot problem areas in porting old JavaScript libraries to Valija.

Neither ES5 not Valija is settled yet, and actual browser behavior is determined only by ongoing experiment, so these lists are preliminary and fluid.

Details

In ES3R but not codified by ES3

  • __proto__

This feature as spelled this way is not expected to ever become standard, since Mozilla uses the double underscore specifically to signal "not standard". In any case, it is not mentionable in Cajita or Valija for that reason. ES5 provides read-only access to an object's prototype by the static reflective method Object.getPrototypeOf(). Once we upgrade Caja to emulate the ES5 APIs, we will probably provide this as well, but currently do not.

  • Function.caller
  • Function.arguments
  • arguments.caller

ES5-non-strict does not specify them. ES5-strict effectively prohibits these by mandating that an attempt to access them throws an error. They currently don't exist in Valija, but we expect to emulate the ES5-strict behavior here as well.

In ES3 or ES3R but not in ES5

  • crazy scoping -- ES3 specifies various gratuitous violations of lexical scoping that are crazy enough that most browsers often ignore the spec language and do something sensible instead.
    • 'as if by the expression "new Array()"' -- if such language is interpreted literally, rebinding or shadowing the Array literal will cause, for example, array literal expressions to use the local binding to Array rather than the original built-in Array constructor.
    • try/catch -- see Deviations document
    • nested function expressions -- as with try/catch, ES3 specifies that the name is brought into scope by adding a new object to the scope chain. Therefore, according to the spec language, properties on Object.prototype will shadow lexical variables of those same name within the body of the nested function.
  • Shared mutable RegExp literal -- ES5 specifies that each evaluation of a RegExp literal expression produce a new RegExp object. Cajita and Valija already implement this.

In ES5, but not in ES5-strict

  • with
  • this-promotion -- when a function mentioning "this" is called as a function, or reflectively with its "thisArg" bound to null, occurrences of "this" in the function become bound instead to the global object. In ES5-strict, such use of "this" will evaluate to undefined. In Valija, "this" will currently evaluate to cajita.USELESS instead.
  • arguments.callee
  • Joining of arguments array with parameter list
  • delete <variable-name>

Taken together, this set of constraints guarantees that scoping in ES5-strict code can be statically analyzed.

The continued adventures of block-nested declarations

  • nested var declarations
  • nested function declarations
  • const declarations
  • let declarations

ES5-strict doesn't provide for any of these; but all are expected in ES-Harmony-strict. Valija allows nested var declarations with the same meaning they have in ES3 and ES5-non-strict.

Valija allows nested function declarations, but not yet with the block-scoped semantics they are expected to have in ES-Harmony. Instead, Valija currently hoists the scoping of the declared function name to the beginning of the enclosing body (as with vars), and hoists the initialization of this variable to a function value to the beginning of the lexically enclosing block. This correctly emulates ES-Harmony's initialization but not its scoping.

In ES3R and ES5-strict, but not in Valija

  • eval()
  • Function() constructor
  • All identifiers ending in a double underscore ("__")
  • Distinction between a function's invoke behavior and its call or apply methods -- In Valija, we translate all calls into calls to the call or apply method of the invoked thing, which need not itself be a function.
  • Function members inherited from Function.prototype --- call, bind, apply, and length are normally inherited from Function.prototype. In Valija, they are on each disfunction instance.

In ES5-strict but not yet in Valija

  • Reflective attribute control -- ES5 adds reflective attribute control operations on the Object() constructor (including the aforementioned Object.getPrototype()).
  • Getters and setters

Summary

Here is a table of features and misfeatures, and the JavaScript family that they are available in:

ES3 ES3R ES5 ES5-strict Cajita Valija
__proto__ Unspecified 3/4 Yes Unspecified Unspecified No No
delete variable Yes Yes Yes No No No
aFunction.caller Unspecified Yes Unspecified No No No
aFunction.arguments Unspecified Yes Unspecified No No No
arguments.caller Unspecified Yes Unspecified No No No
arguments.callee Yes Yes Yes No No No
Modify meaning of [...] by rebinding Array Unclear Varies No No No No
try/catch scope leakage Yes Varies No No No No
named function expression scope leakage Yes Varies No No No No
Leak state between regular expressions Yes Varies No No No No
with Yes Yes Yes No No No
this promotes to global object Yes Yes Yes No No No
Joining arguments with parameter list Yes Yes Yes No No No
eval() Yes Yes Yes Yes1 No No
Function() constructor Yes Yes Yes Yes No No
Identifiers ending in a double underscore ("__") Yes Yes Yes Yes No No
Reflective attribute control No No Yes Yes No No
Getters and setters No 3/4 Yes Yes Yes Future2 Future

1 But without violating static scoping

2 Probably only the get and set syntax in object literals (see Issue 999)

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