Add Compiler Checks For Unused Params And Variables as a typescript til

typescript/compiler-checks-for-unused-params-and-variables.md

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+# Compiler Checks For Unused Params And Variables
+
+There are a number of linter-esque checks that you can tell the TypeScript
+compiler to make when it is checking your code. There are two that prevent
+values from going unused: one for parameters and the other for variables.
+
+The [`noUnusedLocals`](https://www.typescriptlang.org/tsconfig#noUnusedLocals)
+config, which defaults to `false`, can be set to `true`. This will cause the
+compiler to fail if a locally declared variable goes unused.
+
+```typescript
+function printItem(item: any, index: number) {
+  const indexedItem = `${index}: ${item}`;
+  //    ^^^ 'indexedItem' is declared but its value is never read.
+
+  console.log(item);
+}
+```
+
+The
+[`noUnusedParameters`](https://www.typescriptlang.org/tsconfig#noUnusedParameters)
+config, which also defaults to `false`, can be set to `true`. This will cause
+the compiler to fail if a function param goes unused.
+
+Fixing the previous error could then result in this one.
+
+```typescript
+function printItem(item: any, index: number) {
+  //                          ^^^
+  // 'index' is declared but its value is never read.
+
+  console.log(item);
+}
+```
+
+Here is what the `tsconfig.json` would look like:
+
+```json
+{
+  "compilerOptions": {
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+  }
+}
+```