Add normative-conventions.md; document agreed-upon coercion rules

normative-conventions.md

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+# Normative conventions
+
+This document describes some of the conventions that we try to follow when designing new features. Sometimes old features don't follow these; that's ok, new features generally should anyway.
+
+None of these rules are inviolable, but you should have a good reason for any particular feature to deviate (for example, because the feature is a close cousin of an existing feature, like `findLastIndex` vs `findIndex`).
+
+This list is very far from being complete.
+
+## Number-taking inputs should reject `NaN`
+
+In any built-in function which expects a number (including as an option in an options bag), receiving `NaN` or anything which results in `NaN` after coercion is performed (such as by passing through `ToNumeric`, `ToPrimitive`, `ToNumber`, etc.) should cause a `RangeError` to be thrown.
+
+Note that some abstract operations, like [ToIntegerOrInfinity](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tointegerorinfinity), [ToIndex](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-toindex), etc, will coerce inputs to Number and then further coerce the result, sometimes including mapping `NaN` to non-`NaN` values. So you'll need to coerce inputs to a Number using `ToNumber` before calling those operations, check for `NaN`, and then pass the resulting Number to the operation.
+
+An exception to this rule is functions which take optional numeric arguments: in that case, receiving `undefined` should be treated as the argument not being passed.
+
+NB: This convention is new as of 2023, and most earlier parts of the language do not follow it.
+
+## If an argument is expected, not getting it or getting `undefined` should throw a `TypeError`
+
+When there is a required argument, finding that it is explicitly `undefined` or is missing should throw a `TypeError` instead of attempting to coerce `undefined` to the expected type. This extends to values looked up in options bags, etc. This does not apply when there is a default value for the argument, since in that case it is not required.
+
+This convention applies only to `undefined` specifically, and says nothing about the appropriate handling of values like `null` and `document.all`.
+
+NB: This convention is new as of 2023, and most earlier parts of the language do not follow it.