diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts index dd5ad91a70c..e0b565cab03 100644 --- a/src/compiler/checker.ts +++ b/src/compiler/checker.ts @@ -4084,6 +4084,21 @@ module ts { } function isNumericName(name: string) { + // The intent of numeric names is that + // - they are names with text in a numeric form, and that + // - setting properties/indexing with them is always equivalent to doing so with the numeric literal 'numLit', + // acquired by applying the abstract 'ToNumber' operation on the name's text. + // + // The subtlety is in the latter portion, as we cannot reliably say that anything that looks like a numeric literal is a numeric name. + // In fact, it is the case that the text of the name must be equal to 'ToString(numLit)' for this to hold. + // + // Consider the property name '"0xF00D"'. When one indexes with '0xF00D', they are actually indexing with the value of 'ToString(0xF00D)' + // according to the ECMAScript specification, so it is actually as if the user indexed with the string '"61453"'. + // Thus, the text of all numeric literals equivalent to '61543' such as '0xF00D', '0xf00D', '0170015', etc. are not valid numeric names + // because their 'ToString' representation is not equal to their original text. + // This is motivated by ECMA-262 sections 9.3.1, 9.8.1, 11.1.5, and 11.2.1. + // + // Here, we test whether 'ToString(ToNumber(name))' is exactly equal to 'name'. return (+name).toString() === name; }