Typically, implementation-specific things aren't specified in the spec (like Math precision, etc) - although usually when it's implementation-specific, it's explicitly noted as such ( https://people.mozilla.org/~jorendorff/es6-draft.html#sec-date.parse , https://people.mozilla.org/~jorendorff/es6-draft.html#sec-math.hypot , https://people.mozilla.org/~jorendorff/es6-draft.html#sec-ecmascript-language-types-number-type , https://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.keys , etc)
Strings are only defined in ES6 as a "primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer" ( https://people.mozilla.org/~jorendorff/es6-draft.html#sec-terms-and-definitions-string-value ) and are not noted as having any implementation-specific or implementation-dependent qualities. To me, "finite" here means `Number.MAX_VALUE` - ie, the highest number I can get before I reach Infinity. An alternative reading is "any number greater than zero that's not Infinity" - but at that point an implementation conforms if it's max length is 1, which obviously would be silly. However, Chrome 40 and Opera 26-27 have a limit of `0xFFFFFF0` (`2**28 - 2**4`), Firefox 35 and IE 9-11 all have a limit of `0xFFFFFFF` (`2**28 - 1`), and Safari 8 has `0x7FFFFFFF` (`2**31 - 1`). There's many more browsers I haven't tested of course but it'd be interesting to know how wide these numbers deviate. 1) Should an engine's max string length be exposed, like `Number.MAX_VALUE`, as `String.MAX_LENGTH`? This will help, for example, my `String#repeat` polyfill throw an earlier `RangeError` rather than having to try to build a string of that length. 2) Should the spec require a minimum maximum string length, or at least be more specific in how it defines "finite"?
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