How to specify the second string?

This is a terribly stupid question...
If I have stringA "This is a stringA" and stringB "This is also a string"
How do I use the library to diff the 2 strings?
```
let output     = "",
prettydiff = require("prettydiff"),
options    = prettydiff.options;
options.source = stringA;
output         = prettydiff();
```
How do I specify the second string (stringB) to diff against?