Cover union types, type aliases, type guards

pages/Advanced Types.md

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+# Union Types
+
+Occasionally, you'll run into a library that expects or gives back a `number` or `string`.
+For instance, take the following function:
+
+```ts
+/**
+ * Takes a string and adds "padding" to the left.
+ * If 'padding' is a string, then 'padding' is appended to the left side.
+ * If 'padding' is a number, then that number of spaces is added to the left side.
+ */
+function padLeft(value: string, padding: any) {
+    // ...
+}
+
+padLeft("Hello world", 4); // returns "    Hello world"
+```
+
+The problem with `padLeft` is that its `padding` parameter is typed as `any`.
+That means that we can call it with an argument that's neither a `number` nor a `string`, but TypeScript will be okay with it.
+
+```ts
+let indentedString = padLeft("Hello world", true); // passes at compile time, fails at runtime.
+```
+
+In some more traditional object-oriented languages, we might abstract over the two types by creating a hierarchy of types.
+
+```ts
+interface Padder {
+    getPaddingString(): string
+}
+
+class SpaceRepeatingPadder implements Padder {
+    constructor(private numSpaces: number) { }
+    getPaddingString() {
+        return Array(this.numSpaces).join(" ");
+    }
+}
+
+class StringPadder implements Padder {
+    constructor(private value: string) { }
+    getPaddingString() {
+        return this.value;
+    }
+}
+
+function padLeft(value: string, padder: Padder) {
+    return padder.getPaddingString() + value;
+}
+
+padLeft("Hello world", new SpaceRepeatingPadder(4));
+```
+
+While this is much more explicit, it's also a little bit overkill.
+One of the nice things about the original version of `padLeft` was that we were able to just pass in primitives.
+That meant that usage was simple and not overly verbose.
+This new approach also wouldn't help if we were just declaring a function that already exists elsewhere.
+
+Instead of `any`, we can use a *union type* for the `padding` parameter:
+
+```ts
+/**
+ * Takes a string and adds "padding" to the left.
+ * If 'padding' is a string, then 'padding' is appended to the left side.
+ * If 'padding' is a number, then that number of spaces is added to the left side.
+ */
+function padLeft(value: string, padding: string | number) {
+    // ...
+}
+
+let indentedString = padLeft("Hello world", true); // errors during compilation
+```
+
+A union type describes a value that can be one of several types.
+We use the vertical bar (`|`) to separate each type, so `number | string | boolean` is the type of a value that can be a `number`, a `string`, or a `boolean`.
+
+If we have a value that has a union type, we can only access members that are common to all types in the union.
+
+```ts
+interface Bird {
+    fly();
+    layEggs();
+}
+
+interface Fish {
+    swim();
+    layEggs();
+}
+
+function getSmallPet(): Fish | Bird {
+    // ...
+}
+
+let pet = getSmallPet();
+pet.layEggs(); // okay
+pet.swim();    // errors
+```
+
+Union types can be a bit tricky here, but it just takes a bit of intuition to get used to.
+If a value has the type `A | B`, we only know for *certain* that it has members that both `A` *and* `B` have.
+In this example, `Bird` has a member named `fly`.
+We can't be sure whether a variable typed as `Bird | Fish` has a `fly` method.
+If the variable is really a `Fish` at runtime, then calling `pet.fly()` will fail.
+
+# Type Guards and Differentiating Types
+
+Union types are useful for modeling situations when values can overlap in the types they can take on.
+What happens when we need to know specifically whether we have a `Fish`?
+A common idiom in JavaScript to differentiate between two possible values is to check for the presence of a member.
+As we mentioned, you can only access members that are guaranteed to be in all the constituents of a union type.
+
+```ts
+let pet = getSmallPet();
+
+// Each of these property accesses will cause an error
+if (pet.swim) {
+    pet.swim();
+}
+else if (pet.fly) {
+    pet.fly();
+}
+```
+
+To get the same code working, we'll need to use a type assertion:
+
+```ts
+let pet = getSmallPet();
+
+if ((<Fish>pet).swim) {
+    (<Fish>pet).swim();
+}
+else {
+    (<Bird>pet).fly();
+}
+```
+
+## User-Defined Type Guards
+
+Notice that we had to use type assertions several times.
+It would be much better if once we performed the check, we could know the type of `pet` within each branch.
+
+It just so happens that TypeScript has something called a *type guard*.
+A type guard is some expression that performs a runtime check that guarantees the type in some scope.
+To define a type guard, we simply need to define a function whose return type is a *type predicate*:
+
+```ts
+function isFish(pet: Fish | Bird): pet is Fish {
+    return (<Fish>pet).swim !== undefined;
+}
+```
+
+`pet is Fish` is our type predicate in this example.
+A predicate takes the form `parameterName is Type`, where `parameterName` must be the name of a parameter from the current function signature.
+
+Any time `isFish` is called with some variable, TypeScript will *narrow* that variable to that specific type if the original type is compatible.
+
+```ts
+// Both calls to 'swim' and 'fly' are now okay.
+
+if (isFish(pet) {
+    pet.swim();
+}
+else {
+    pet.fly();
+}
+```
+
+Notice that TypeScript not only knows that `pet` is a `Fish` in the `if` branch;
+it also knows that in the `else` branch, you *don't* have a `Fish`, so you must have a `Bird`.
+
+## `typeof` type guards
+
+We didn't actually reveal the implementation of the version of `padLeft` which used union types.
+We could write it with type predicates as follows:
+
+```ts
+function isNumber(x: any): x is number {
+    return typeof x === "number";
+}
+
+function isString(x: any): x is string {
+    return typeof x === "string";
+}
+
+function padLeft(value: string, padding: string | number) {
+    if (isNumber(padding)) {
+        return Array(padding).join(" ") + value;
+    }
+    if (isString(padding)) {
+        return padding + value;
+    }
+    throw new Error(`Expected string or number, got '${value}'.`);
+}
+```
+
+However, having to define a function to figure out if a type is a primitive is kind of a pain.
+Luckily, you don't need to abstract `typeof x === "number"` into its own function because TypeScript will recognize it as a type guard on its own.
+That means we could just write these checks inline.
+
+```ts
+function padLeft(value: string, padding: string | number) {
+    if (typeof padding === "number") {
+        return Array(padding).join(" ") + value;
+    }
+    if (typeof padding === "string") {
+        return padding + value;
+    }
+    throw new Error(`Expected string or number, got '${value}'.`);
+}
+```
+
+These *`typeof` type guards* are recognized in two different forms: `typeof v === "typename"` and `typeof v !== "typename"`, where `"typename"` must be `"number"`, `"string"`, `"boolean"`, or `"symbol"`.
+While TypeScript won't prohibit using a string other than the aforementioned ones, or switching the two sides of the comparison, the language won't recognize those forms as type guards.
+
+## `instanceof` type guards
+
+If you've read about `typeof` type guards and are familiar with the `instanceof` operator in JavaScript, you probably have some idea of what this section is about.
+
+*`instanceof` type guards* are a way of narrowing types using their constructor function.
+For instance, let's borrow our industrial string-padder example from earlier:
+
+```ts
+interface Padder {
+    getPaddingString(): string
+}
+
+class SpaceRepeatingPadder implements Padder {
+    constructor(private numSpaces: number) { }
+    getPaddingString() {
+        return Array(this.numSpaces).join(" ");
+    }
+}
+
+class StringPadder implements Padder {
+    constructor(private value: string) { }
+    getPaddingString() {
+        return this.value;
+    }
+}
+
+function getRandomPadder() {
+    return Math.random() < 0.5 ?
+        new SpaceRepeatingPadder(4) :
+        new StringPadder("  ");
+}
+
+// Type is SpaceRepeatingPadder | StringPadder
+let padder: Padding = getRandomPadder();
+
+if (padder instanceof SpaceRepeatingPadder) {
+    padder; // type narrowed to 'SpaceRepeatingPadder'
+}
+if (padder instanceof StringPadder) {
+    padder; // type narrowed to 'StringPadder'
+}
+```
+
+The right side of the `instanceof` needs to be a constructor function, and TypeScript will narrow down to:
+
+1. the type of the function's `prototype` property if its type is not `any`
+2. the union of types returned by that type's construct signatures
+
+in that order.
+
+# Type Aliases
+
+Type aliases create a new name for a type.
+Type aliases are sometimes similar to interfaces, but can name primitives, unions, tuples, and any other types that you'd otherwise have to write by hand.
+
+```ts
+type XCoord = number;
+type YCoord = number;
+
+type XYCoord = { x: XCoord; y: YCoord };
+type XYZCoord = { x: XCoord; y: YCoord; z: number };
+
+type Coordinate = XCoord | XYCoord | XYZCoord;
+type CoordList = Coordinate[];
+
+let coord: CoordList = [{ x: 10, y: 10}, { x: 0, y: 42, z: 10 }, { x: 5 }];
+```
+
+Aliasing doesn't actually create a new type - it creates a new *name* to refer to that type.
+So `10` is a perfectly valid `XCoord` and `YCoord` because they both just refer to `number`.
+Aliasing a primitive is not terribly useful, though it can be used as a form of documentation.
+
+Just like interfaces, type aliases can also be generic - we can just add type parameters and use them on the right side of the alias declaration:
+
+```ts
+type Container<T> = { value: T };
+```
+
+We can also have a type alias refer to itself in a property:
+
+```ts
+type Tree<T> = {
+    value: T;
+    left: Tree<T>;
+    right: Tree<T>;
+}
+```
+
+However, it's not possible for a type alias to appear anywhere else on the right side of the declaration:
+
+```ts
+type Yikes = Array<Yikes>; // error
+```
+
+## Interfaces vs. Type Aliases
+
+As we mentioned, type aliases can act sort of like interfaces; however, there are some subtle differences.
+
+One important difference is that type aliases cannot be extended or implemented from (nor can they extend/implement other types).
+Because [an ideal property of software is being open to extension](https://en.wikipedia.org/wiki/Open/closed_principle), you should always use an interface over a type alias if possible.
+
+On the other hand, if you can't express some shape with an interface and you need to use a union or tuple type, type aliases are usually the way to go.