Why 42.toString() fails in JS?

问题

Disclaimer

Guys, I DO aware of Why does 10..toString() work, but 10.toString() does not? question existence, but the thing is that it doesn't provide the formal explanation.

The specification's interpretation of the . character in that particular position is that it will be a decimal. This is defined by the numeric literal syntax of ECMAScript.

Without reference to a standard isn't trustable enough

The question body

I subconsciously understand that

42..toString()

is treated by a parser as a 42. number followed by a .toString() call.

What I cannot understand is why an interpreter cannot realize that

42.toString()

is a 42 followed by a method call.

Is it just a drawback of modern JS interpreters or is it explicitly stated by ES5.1?

From ES5.1 the Numeric Literal is defined as (only significant part of definition):

NumericLiteral ::
    DecimalLiteral
    HexIntegerLiteral

DecimalLiteral ::
    DecimalIntegerLiteral . DecimalDigits(opt) ExponentPart(opt)
    . DecimalDigits ExponentPart(opt)
    DecimalIntegerLiteral ExponentPart(opt)

The last rule is what I expect to be chosen by a parser.

UPD: to clarify, this question expects as an answer references to ES specification that state explicitly that interpreter must behave like it does

回答1:

I believe the piece you're missing is this quote from section 7:

The source text is scanned from left to right, repeatedly taking the longest possible sequence of characters as the next input element.

Note "longest possible sequence of characters"; since "42." is a valid token (which is a kind of input element), it must be used rather than "42" and then ".".

回答2:

The lexical phase of the parsing process consumes the . precisely due to the definition of NumericLiteral quoted from the standard. The definition specifies that a numeric literal (here a DecimalLiteral) includes a ., if present - see the first line under DecimalLiteral ::.

Since 42. is consumed by the numeric literal token, the next token is toString, which lacks a leading period and therefore cannot be recognized as method call.

来源：https://stackoverflow.com/questions/24398347/why-42-tostring-fails-in-js

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javascript

ecmascript-5