fix ns.prompt typechecking

Introduced some new type assertion functions, initial step in larger type safety rework.

src/utils/helpers/typeAssertion.ts

@@ -0,0 +1,42 @@
+// Various functions for asserting types.
+
+/** Function for providing custom error message to throw for a type assertion.
+ * @param v: Value to assert type of
+ * @param assertFn: Typechecking function to use for asserting type of v.
+ * @param msgFn: Function to use to generate an error message if an error is produced. */
+export function assert<T>(
+  v: unknown,
+  assertFn: (v: unknown) => asserts v is T,
+  msgFn: (type: string) => string,
+): asserts v is T {
+  try {
+    assertFn(v);
+  } catch (type: unknown) {
+    if (type !== "string") type = "unknown";
+    throw msgFn(type as string);
+  }
+}
+
+/** Returns the friendlyType of v. arrays are "array" and null is "null". */
+export function getFriendlyType(v: unknown): string {
+  return v === null ? "null" : Array.isArray(v) ? "array" : typeof v;
+}
+
+//All assertion functions used here should return the friendlyType of the input.
+
+/** For non-objects, and for array/null, throws the friendlyType of v. */
+export function objectAssert(v: unknown): asserts v is Partial<Record<string, unknown>> {
+  const type = getFriendlyType(v);
+  if (type !== "object") throw type;
+}
+
+/** For non-string, throws the friendlyType of v. */
+export function stringAssert(v: unknown): asserts v is string {
+  const type = getFriendlyType(v);
+  if (type !== "string") throw type;
+}
+
+/** For non-array, throws the friendlyType of v. */
+export function arrayAssert(v: unknown): asserts v is unknown[] {
+  if (!Array.isArray(v)) throw getFriendlyType(v);
+}