javascript - JavaScript 是否有未定义的行为？

Question

JavaScript 是否具有未定义的行为(类似于 C)，或者它是否完全由规范定义且具有确定性？

请注意，我正在丢弃实现错误和规范分歧。我也在丢弃 Math.random() 之类的东西和 Date.now() .

是否有一段 JavaScript 代码的行为未完全由 JavaScript 规范确定，并且因此具有“未定义的行为”？

Answer 1

规范中有很多东西明确地留给了实现。尤其是当涉及到主机对象时，可能会有很多怪癖。与宿主对象无关的示例:

15.1 The Global Object

The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.

15.1.2.2 parseInt (string , radix)

[If there are too many significant digits] mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.

15.3.4.2 Function.prototype.toString

An implementation-dependent representation of the function is returned.

几乎所有的日期解析/字符串化算法都是 依赖于实现 , 这包括 toLocaleString , toString , parse和 Date构造函数。

15.4.4.11 Array.prototype.sort (comparefn) - likely the best example:

If comparefn is not undefined and is not a consistent comparison function for the elements of this array, the behaviour of sort is implementation-defined.

[…] If proto is not null and there exists an integer j such that all of the conditions below are satisfied then the behaviour of sort is implementation-defined:

• obj is sparse (15.4)
• 0 ≤ j < len

The behaviour of sort is also implementation defined if obj is sparse and any of the following conditions are true:

• The [[Extensible]] internal property of obj is false.
• Any array index property of obj whose name is a nonnegative integer less than len is a data property whose [[Configurable]] attribute is false.

The behaviour of sort is also implementation defined if any array index property of obj whose name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute is false.

最突出的是:

Perform an implementation-dependent sequence of calls […]

15.5.4.9 String.prototype.localeCompare (that)

The two Strings are compared in an implementation-defined fashion

15.5.4.11 String.prototype.replace[In replacement symbols, if the number is greater than the number of groups], the result is implementation-defined.

我将在此处停止列出，您可以通过规范搜索。其他值得注意的地方可能是 toLocaleString方法，或 Math 返回的依赖于实现的近似值方法。