Add fs.ts to abstract over fs

src/fs.ts

@@ -0,0 +1,35 @@
+/**
+ * This module exports a `FileSystem` type which corresponds to the interface of the "fs" module.
+ *
+ * Functions which are parameterised over this type can then be passed a different implementation in tests:
+ *
+ * ```typescript
+ * import * as nodefs from "fs";
+ *
+ * function foo(fs: FileSystem = nodefs) {
+ *   // Uses the real "fs" module by default, but can be given a different implementation.
+ * }
+ * ```
+ *
+ * The type can also be constrained to a subset of available operations. For example, in the following
+ * case we have a function that only needs `statSync`:
+ *
+ * ```
+ * function bar(fs: FileSystem<"statSync"> = nodefs) {
+ *   // This function can only use `statSync`.
+ * }
+ * ```
+ *
+ * This is useful to define a clearer interface for what the function does and also only requires stubbing
+ * of the relevant functions.
+ */
+
+import * as fs from "fs";
+
+/** Represents the names of operations exported from "fs". */
+export type FileOperation = keyof typeof fs;
+
+/** Represents the type of "fs", optionally filtered down to just `Ops`. */
+export type FileSystem<Ops extends FileOperation = keyof typeof fs> = {
+  [Key in Ops]: (typeof fs)[Key];
+};