Edit on GitHubReScript compiles to JavaScript ES5, with the exception of optionally allowing to compile to ES6's module import & export.
For old browsers, you also need to polyfill TypedArray. The following standard library functions require it:
Int64.float_of_bitsInt64.bits_of_floatInt32.float_of_bitsInt32.bits_of_float
If you don't use these functions, you're fine. Otherwise, it'll be a runtime failure.