An OPC-UA client in Javascript for a browser is useful on a desktop client with strong computing power ( i7 with 4 CPU cores, 8 threads..), less with clients with limited computing power ( e.g. a webbook, an ARM A9 based embedded system, an ARM based smartphone,.. ).
Binary data processing with Javascript:
Arraybuffer.
TypedArray ( ArrayBufferView ).
DataViewe ( ArrayBufferView ).
Blob.
Javascript can handle 64-bit integer values just by special Javascript libraries, which handle 64-bit values as strings.
CommonJS Wiki "Promises" - “A well-defined interface for interacting with an object that represents the result of an action that is performed asynchronously”.