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`let` is not yet part of the current ECMAScript standard but is part of the ECMAScript 6 draft and is supported by Mozilla, and people already use it in some contexts. Also, the current ECMAScript standard marks `let` as a "considered reserved word" (7.6.1.2), so it is already a reserved keyword in strict mode. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let