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Disallows comparing the result of String.prototype.charAt() against a string whose length is not 1.

charAt always returns a string of length 1 (a single UTF-16 code unit), or an empty string when the index is out of range. Comparing it against a string of length 2 or more can therefore never be true, which is almost always a bug. Escape sequences such as '\n' are a single character and are fine.

Invalid:

a.charAt(4) === "a2";
a.charAt(4) === "/n";

Valid:

a.charAt(4) === "a";
a.charAt(4) === "\n";