Question

1.EC6 保留字（Reserved keywords as of ECMAScript 6） https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Keywords

2.ECMAScript 的严格模式(The Strict Mode of ECMAScript) http://es5.github.io/#C 或 https://msdn.microsoft.com/en-us/library/br230269(v=vs.94).aspx