Fixed 221 exercise and improved type predicates section

Author: mattpocock

book-content/chapters/16-the-utils-folder.md

@@ -424,7 +424,20 @@ function isAlbum(input: unknown) {
 }
 ```
 
-This can feel far too verbose. We can make it more readable by adding our own type predicate.
+But at this point, something frustrating happens - TypeScript _stops_ inferring the return value of the function. We can see this by hovering over `isAlbum`:
+
+```typescript
+// hovering over isAlbum shows:
+function isAlbum(input: unknown): boolean;
+```
+
+This is because TypeScript's type predicate inference has limits - it can only process a certain level of complexity.
+
+Not only that, but our code is now _extremely_ defensive. We're checking the existence _and_ type of every property. This is a lot of boilerplate, and might not be necessary. In fact, code like this should probably be encapsulated in a library like [Zod](https://zod.dev/).
+
+### Writing Your Own Type Predicates
+
+To solve this, we can manually annotate our `isAlbum` function with a type predicate:
 
 ```typescript
 function isAlbum(input: unknown): input is Album {
@@ -439,7 +452,9 @@ function isAlbum(input: unknown): input is Album {
 }
 ```
 
-Now, when we use `isAlbum`, TypeScript will know that the type of the value has been narrowed to `Album`:
+This annotation tells TypeScript that when `isAlbum` returns `true`, the type of the value has been narrowed to `Album`.
+
+Now, when we use `isAlbum`, TypeScript will infer it correctly:
 
 ```typescript
 const run = (maybeAlbum: unknown) => {
@@ -449,19 +464,21 @@ const run = (maybeAlbum: unknown) => {
 };
 ```
 
-For complex type guards, this can be much more readable.
+This can ensure that you get the same type behavior from complex type guards.
 
 ### Type Predicates Can be Unsafe
 
-Authoring your own type predicates can be a little dangerous. If the type predicate doesn't accurately reflect the type being checked, TypeScript won't catch that discrepancy:
+Authoring your own type predicates can be a little dangerous. TypeScript doesn't track if the type predicate's runtime behavior matches the type predicate's type signature.
 
 ```typescript
-function isAlbum(input): input is Album {
-  return typeof input === "object";
+function isNumber(input: unknown): input is number {
+  return typeof input === "string";
 }
 ```
 
-In this case, any object passed to `isAlbum` will be considered an `Album`, even if it doesn't have the required properties. This is a common pitfall when working with type predicates - it's important to consider them about as unsafe as `as` and `!`.
+In this case, TypeScript _thinks_ that `isNumber` checks if something is a number. But in fact, it checks if something is a string! There are no guarantees that the runtime behavior of the function matches the type signature.
+
+This is a common pitfall when working with type predicates - it's important to consider them about as unsafe as `as` and `!`.
 
 ## Assertion Functions
 

src/085-the-utils-folder/221-type-predicates.problem.ts

@@ -1,16 +1,68 @@
 import { Equal, Expect } from "@total-typescript/helpers";
-import { expect, it } from "vitest";
+import { describe, expect, it } from "vitest";
 
-const isString = (input: unknown) => {
-  return typeof input === "string";
+const hasDataAndId = (value: unknown) => {
+  return (
+    typeof value === "object" &&
+    value !== null &&
+    "data" in value &&
+    typeof value.data === "object" &&
+    value.data !== null &&
+    "id" in value.data &&
+    typeof value.data.id === "string"
+  );
 };
 
-it("Should be able to be passed to .filter and work", () => {
-  const mixedArray = [1, "hello", [], {}];
+const parseValue = (value: unknown) => {
+  if (hasDataAndId(value)) {
+    return value.data.id;
+  }
 
-  const stringsOnly = mixedArray.filter(isString);
+  throw new Error("Parsing error!");
+};
+
+const parseValueAgain = (value: unknown) => {
+  if (hasDataAndId(value)) {
+    return value.data.id;
+  }
+
+  throw new Error("Parsing error!");
+};
+
+describe("parseValue", () => {
+  it("Should handle a { data: { id: string } }", () => {
+    const result = parseValue({
+      data: {
+        id: "123",
+      },
+    });
+
+    type test = Expect<Equal<typeof result, string>>;
+
+    expect(result).toBe("123");
+  });
+
+  it("Should error when anything else is passed in", () => {
+    expect(() => parseValue("123")).toThrow("Parsing error!");
+    expect(() => parseValue(123)).toThrow("Parsing error!");
+  });
+});
+
+describe("parseValueAgain", () => {
+  it("Should handle a { data: { id: string } }", () => {
+    const result = parseValueAgain({
+      data: {
+        id: "123",
+      },
+    });
+
+    type test = Expect<Equal<typeof result, string>>;
 
-  type test1 = Expect<Equal<typeof stringsOnly, string[]>>;
+    expect(result).toBe("123");
+  });
 
-  expect(stringsOnly).toEqual(["hello"]);
+  it("Should error when anything else is passed in", () => {
+    expect(() => parseValueAgain("123")).toThrow("Parsing error!");
+    expect(() => parseValueAgain(123)).toThrow("Parsing error!");
+  });
 });

src/085-the-utils-folder/221-type-predicates.solution.ts

@@ -1,16 +1,74 @@
 import { Equal, Expect } from "@total-typescript/helpers";
-import { expect, it } from "vitest";
+import { describe, expect, it } from "vitest";
 
-const isString = (input: unknown): input is string => {
-  return typeof input === "string";
+const hasDataAndId = (
+  value: unknown,
+): value is {
+  data: {
+    id: string;
+  };
+} => {
+  return (
+    typeof value === "object" &&
+    value !== null &&
+    "data" in value &&
+    typeof value.data === "object" &&
+    value.data !== null &&
+    "id" in value.data &&
+    typeof value.data.id === "string"
+  );
 };
 
-it("Should be able to be passed to .filter and work", () => {
-  const mixedArray = [1, "hello", [], {}];
+const parseValue = (value: unknown) => {
+  if (hasDataAndId(value)) {
+    return value.data.id;
+  }
 
-  const stringsOnly = mixedArray.filter(isString);
+  throw new Error("Parsing error!");
+};
+
+const parseValueAgain = (value: unknown) => {
+  if (hasDataAndId(value)) {
+    return value.data.id;
+  }
+
+  throw new Error("Parsing error!");
+};
+
+describe("parseValue", () => {
+  it("Should handle a { data: { id: string } }", () => {
+    const result = parseValue({
+      data: {
+        id: "123",
+      },
+    });
+
+    type test = Expect<Equal<typeof result, string>>;
+
+    expect(result).toBe("123");
+  });
+
+  it("Should error when anything else is passed in", () => {
+    expect(() => parseValue("123")).toThrow("Parsing error!");
+    expect(() => parseValue(123)).toThrow("Parsing error!");
+  });
+});
+
+describe("parseValueAgain", () => {
+  it("Should handle a { data: { id: string } }", () => {
+    const result = parseValueAgain({
+      data: {
+        id: "123",
+      },
+    });
+
+    type test = Expect<Equal<typeof result, string>>;
 
-  type test1 = Expect<Equal<typeof stringsOnly, string[]>>;
+    expect(result).toBe("123");
+  });
 
-  expect(stringsOnly).toEqual(["hello"]);
+  it("Should error when anything else is passed in", () => {
+    expect(() => parseValueAgain("123")).toThrow("Parsing error!");
+    expect(() => parseValueAgain(123)).toThrow("Parsing error!");
+  });
 });