Improve error handling in case of illegal sequence input

Author: winterbe

README.md

@@ -14,7 +14,7 @@
 
 ## About Sequency
 
-Sequency is a lightweight (**3 KB minified**), intensely tested (**175+ tests, 99% coverage**), type-safe functional programming library for processing iterable data such as arrays, sets and maps. It's written in TypeScript, compiles to ES5-compatible JavaScript and works in all current browsers and Node applications. The API is inspired by [Sequences](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/-sequence/) from the programming language [Kotlin](https://kotlinlang.org/).
+Sequency is a lightweight (**3 KB minified**), intensely tested (**200+ tests, 99% coverage**), type-safe functional programming library for processing iterable data such as arrays, sets and maps. It's written in TypeScript, compiles to ES5-compatible JavaScript and works in all current browsers and Node applications. The API is inspired by [Sequences](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/-sequence/) from the programming language [Kotlin](https://kotlinlang.org/).
 
 > Not convinced? [Try Sequency](https://npm.runkit.com/sequency) right in your browser.
 

src/Sequence.ts

@@ -165,6 +165,15 @@ export function emptySequence<T>(): Sequence<T> {
 }
 
 export function asSequence<T>(iterable: Iterable<T>): Sequence<T> {
+    if (iterable === null) {
+        throw new Error("Cannot create sequence for input: null");
+    }
+    if (iterable === undefined) {
+        throw new Error("Cannot create sequence for input: undefined");
+    }
+    if (iterable[Symbol.iterator] == null) {
+        throw new Error("Cannot create sequence for non-iterable input: " + iterable);
+    }
     return new Sequence<T>(new IterableIterator(iterable));
 }
 

test/asSequence.test.ts

@@ -58,4 +58,22 @@ describe("asSequence", () => {
         expect(array[1]).toEqual(["b", 2]);
         expect(array[2]).toEqual(["c", 3]);
     });
+
+    it("should throw understandable error message if input is undefined", () => {
+        expect(
+            () => asSequence(undefined as Array<number>).toArray()
+        ).toThrowError("Cannot create sequence for input: undefined");
+    });
+
+    it("should throw understandable error message if input is null", () => {
+        expect(
+            () => asSequence(null as Array<number>).toArray()
+        ).toThrowError("Cannot create sequence for input: null");
+    });
+
+    it("should throw understandable error message if input is not iterable", () => {
+        expect(
+            () => asSequence({} as Array<number>).toArray()
+        ).toThrowError("Cannot create sequence for non-iterable input: [object Object]");
+    });
 });