mongolian vowel separator
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There was a backwards-incompatible change in ES7 corresponding to whitespace, as the Unicode version pin was removed:
https://www.ecma-international.org/ecma-262/6.0/#sec-white-space
ECMAScript implementations must recognize as WhiteSpace code points listed in the “Separator, space” (Zs) category by Unicode 5.1.
https://www.ecma-international.org/ecma-262/7.0/#sec-white-space
ECMAScript implementations must recognize as WhiteSpace code points listed in the “Separator, space” (Zs) category.
The MONGOLIAN VOWEL SEPARATOR U+180E used to be in Zs but was moved to Cf in Unicode 6.3.0. ES7 mandates Unicode 8.0.0 or higher. As a result, whitespace-sensitive methods will behave differently.
For example, consider "\u180e".trim().length. An ES6-compatible browser like Safari 11 will remove the character, yielding a length of 0. However, an ES7-compatible engine should not remove the character, yielding a length of 1.
String#trim and other methods that lean on whitespace are affected.
tc39/ecma262#1698 corrects this omission in the spec's Annex E.