The 2019 paper[1] says: “STS primitive types are treated according to JavaScript semantics. In particulars, all numbers are logically IEEE 64-bit floating point, but 31-bit signed tagged integers are used where possible for performance. Implementation of operators, like addition or comparison, branch on the dynamic types of values to follow JavaScript semantics[.]”

[1]: https://www.microsoft.com/en-us/research/publication/static-...