Updates, completing incomplete sentences

Author: threehams

README.md

@@ -4,22 +4,40 @@ TypeScript makes some normally complex tasks very easy, but it can make some see
 
 ## Some things that were hard: now easy!
 
-Change an API? Change the type and you'll see everywhere you need to update. Need to rename something used in 500 places? Hit F2 and rename. Anything requiring fle
+Change an API? Change the type and you'll see everywhere you need to update. Need to rename something used in 500 places? Hit F2 and rename.
 
 ## Some things that were easy: now hard.
 
-TypeScript actively discourages highly-dynamic code, redefining variables to mean different things, and anything other than simple (property) strings for APIs. What do these look like?
+TypeScript actively discourages highly-dynamic code, redefining variables at will, and APIs that put a lot of meaning into strings. What do these look like?
 
 ```tsx
-// Dynamic code:
+const sizes = {
+  "size-sm": 1,
+  "size-md": 2,
+  "size-lg": 3,
+};
+
+const getSize = (size: "sm" | "md" | "lg") => {
+  return sizes[`size-${size}`];
+};
 ```
 
+This site is all about letting you know about these limitations _without_ all the type-system jargon you'll normally see.
+
+# Diagnostic Codes
+
+TypeScript has a code for every error, such as `ts(2741)`. While the error message may look very different each time, the underlying problem is the same.
+
+If you're hitting a problem, search for that code within this page.
+
 # For Site / Application Developers
 
 ## I'm trying to start with an empty object and add properties one by one, and TypeScript gives me errors no matter what I do. I've given up and `// @ts-ignore`d them.
 
 When you create an empty object, there's just no good way for TS to ensure that all properties have been added _after_ the object has already been specified. Don't worry, though - there are lots of ways to accomplish this, and one of them is very concise as well! See: [Build Up Object](examples/build-up-object.ts).
 
+Diagnostic codes: `ts(2339)`, `ts(2741)`
+
 ## What's this `never` type?
 
 This is covered in the new handbook under the [never type](https://microsoft.github.io/TypeScript-New-Handbook/chapters/more-on-functions/#never).

examples/limitations.ts

@@ -2,6 +2,7 @@ export {};
 
 // go over APIs which can't be modeled at all
 // example: camelize
+// issue: https://github.com/microsoft/TypeScript/issues/12754
 () => {
   // from: https://stackoverflow.com/questions/2970525/converting-any-string-into-camel-case
   const camelize = (str: string) => {

examples/never.ts

@@ -19,11 +19,18 @@ function fail(msg: string): never {
 }
 
 // The other time you'll see `never` is when creating a new empty array, when
-// noImplicityAny is set to `false`. It is extremely unlikely you will see this,
-// but the type defaults to `never[]` to prevent it from being inferred as `any[]`.
+// noImplicityAny is set to `false`. It is unlikely you will see this, but the
+// type defaults to `never[]` to prevent it from being inferred as `any[]`.
 // Simple fix: give it a type, or set `noImplicitAny` to true.
 // https://github.com/microsoft/TypeScript/pull/8944
 
-const emptyArray = []; // never[]
-emptyArray.push(1); // nope!
-const emptyNumberArray: number[] = [];
+// Sometimes `never[]` is really a symptom of another error. Here's a case where
+// an object is
+
+type Resolver = {
+  location: { city: string; state: string };
+};
+
+const resolver: Resolver = {
+  location: [],
+};

examples/safe-property-access.ts

@@ -91,10 +91,6 @@ const doStuffWithUnions = (data?: UnreliableUnionData) => {
   oc(structure as StructureOne).deepMayExist.nullableNumber(0);
 };
 
-// Destructuring with an empty object by default can result in confusing errors:
 const doStuffWithDefaults = (data: UnreliableData) => {
-  // "Initializer provides no value for this binding element and the binding element has no default value.ts(2525)"
-  const { nullableString }: UnreliableData["mayExist"] = data.mayExist || {};
-  // This will be fixed when TypeScript 3.6 is released:
-  // https://github.com/microsoft/TypeScript/pull/31711
+  const { deepMayExist } = data.mayExist || {};
 };

examples/type-widening.ts

@@ -6,7 +6,7 @@ export {};
 
 // tl;dr rules for avoiding this:
 // 1. never reassign parameters. https://eslint.org/docs/rules/no-param-reassign
-// 2.
+// 2. use `const` unless you need `let` (enable the prefer-const eslint rule)
 
 const maybeAsync = (callback: () => void) => {
   callback();

package.json

@@ -10,19 +10,18 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
-    "@types/lodash": "^4.14.135",
-    "@types/react": "^16.8.13",
-    "@typescript-eslint/eslint-plugin": "^1.6.0",
-    "@typescript-eslint/parser": "^1.6.0",
-    "eslint": "^5.16.0",
-    "eslint-plugin-jsx-a11y": "^6.2.1",
-    "eslint-plugin-react": "^7.12.4",
-    "eslint-plugin-react-hooks": "^1.6.0",
-    "lodash": "^4.17.13",
-    "react": "^16.8.6",
-    "redux": "^4.0.1",
+    "@types/lodash": "^4.14.149",
+    "@types/react": "^16.9.16",
+    "@typescript-eslint/eslint-plugin": "^2.11.0",
+    "@typescript-eslint/parser": "^2.11.0",
+    "eslint": "^6.7.2",
+    "eslint-plugin-jsx-a11y": "^6.2.3",
+    "eslint-plugin-react": "^7.17.0",
+    "eslint-plugin-react-hooks": "^2.3.0",
+    "lodash": "^4.17.15",
+    "react": "^16.12.0",
+    "redux": "^4.0.4",
     "redux-thunk": "^2.3.0",
-    "ts-optchain": "^0.1.4",
-    "typescript": "^3.4.2"
+    "typescript": "^3.7.3"
   }
 }