fix: currency data type

[gantunesr]

Explanation

There's an inconsistency regarding the currency rate data type, this PR changes it from string to number

References

Changelog

@metamask/assets-controller

• BREAKING: RatesController data type update.
  • Change data type to number of conversionRate and usdConversionRate from Rate

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

[legobeat]

It looks like string is a good initial choice here to preserve precision and accuracy which gets lost when converting the value to floating point number?

Is there some reason why continuing to return strings is problematic here? If so, could we consider a precise decimal type (e.g. BigNumber)?

Nice properties of decimal strings is that they are (relatively) non-ambiguous, serialize cleanly to/from JSON, and don't rely on any specific implementations.

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I see that CurrencyRateState is still handling and exposing rates as numbers - should we consider doing the inverse and changing that instead?

[gantunesr]

I agree with your statement @legobeat. This was the reason I did it as string in the first place and later I decided to make this change because we only use this value for UI/UX purposes and not for any calculation. I'll close this PR if nobody opposes to it @ccharly @GuillaumeRx

[GuillaumeRx]

I don't mind returning a string, I think this is a good technical choice. The only problem here is that the API returns a number and it is never stringified. We should stringify the return value if we want to keep the string type @gantunesr @legobeat