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Use Number.p.toExponential for finite string values, Number for non-finite values #86

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Use Number.p.toExponential for finite string values, Number for non-finite values #86

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23 changes: 15 additions & 8 deletions README.md
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Expand Up @@ -47,13 +47,19 @@ Amount will have the following read-only properties:
Note: ⚠️ All property/method names up for bikeshedding.

* `value` (Number or BigInt or String): The numerical value of the amount.
By default, the type of the value used in the constructor is retained.
The value of an Amount constructed with precision options,
or one that's the result of unit conversion,
is always a numerical string.
The type of the value used in the constructor is retained,
except that any non-finite value is a Number (`Infinity`, `-Infinity`, or `NaN`)
and any value that was potentially affected by precision options and/or unit conversion is a String.

A String `value` is always in the format returned by [Number.p.toExponential]
(decimal exponential notation with an explicitly signed exponent
and a significand that is either exactly 0 or has a positive absolute value less than 10)
but not necessarily limited by the same bounds.
* `unit` (String or not defined): The unit of measurement associated with the Amount's numerical value.
An undefined value indicates "no unit supplied".

[Number.p.toExponential]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toExponential

### Constructor

* `new Amount(value[, options])`. Constructs an Amount with the numerical value of `value`
Expand All @@ -67,11 +73,11 @@ Note: ⚠️ All property/method names up for bikeshedding.
When constructing an Amount from a String `value`,
its mathematical value is parsed using [StringNumericLiteral](https://tc39.es/ecma262/#prod-StringNumericLiteral)
or a RangeError is thrown.
The `value` property of a String-valued Amount is not necessarily equal to the `value` its constructor was called with,
as it is always a [StrDecimalLiteral](https://tc39.es/ecma262/#prod-StrDecimalLiteral), or `"NaN"`.
The `value` property of a String-valued Amount is always normalized to decimal exponential notation as described above.

If either `fractionDigits` or `significantDigits` is set,
the `value` is rounded accordingly, and is stored as a String.
the `value` is rounded accordingly,
and is stored as a String (if finite) or Number (if not finite).

The object prototype would provide the following methods:

Expand Down Expand Up @@ -102,7 +108,8 @@ The object prototype would provide the following methods:
`{ minimumFractionDigits: 0, maximumFractionDigits: 3}` is used.
If both fraction and significant digit options are set,
the resulting behaviour is selected by the `roundingPriority`.
The numerical value of the Amount resulting from unit conversion is stored as a String.
The numerical value of the Amount resulting from unit conversion is
stored as a String (if finite) or Number (if not finite).

Calling `convertTo()` will throw an error if conversion is not supported
for the Amount's unit (such as currency units),
Expand Down