Update doc

doc/TS-MIGRATION.md

@@ -8,6 +8,30 @@ Good examples of data are option objects to have named parameters, and POJO (pla
 
 Also see [this playground](https://www.typescriptlang.org/play?#code/C4TwDgpgBACghgJwgO2AeTMAlge2QZygF4oBvAKCiqmTgFsIAuKfYBLZAcwG5LqATCABs4IAPzNkAVzoAjCAl4BfcuVCQoAYQAWWIfwzY8hEvCSpDuAlABkZPlQDGOITgTNW7LstWOR+QjMUYHtqKGcCNilHYDcAChxMK3xmIIsk4wBKewcoFRVyPzgArV19KAgAD2AUfkDEYNDqCM9o2IQEjIJmHT0DLvxsijCw-ClIDsSjAkzeEebjEIYAuE5oEgADABJSKeSAOloGJSgsQh29433nVwQlDbnqfKA)
 
-## Use `Record<string, any>` to describe a type that accepts any Javascript object.
+## Use `type foo = { [key: string]: any }` for types that you intend to fill in later.
 
-`Record<string, any>` allows us to avoid passing in primitive types and prevents type errors when accessing properties. As this is a temporary type while converting javascript, it seems best to not add any additional requirements. If any errors occur, they must have already been present, and we should fix it by adding proper types. So prefer `Record<string, any>` over `[key: string]: any`, `any` or `object`.
+For instance, if you have a method such as:
+```js
+    function load(options) {
+        // ...
+    }
+```
+and you intend to type options at some later point, do:
+```ts
+    type Options = { [key: string]: any}
+```
+This makes it much easier to add the necessary type information at a later time.
+
+## Use `object` or `Record<string, any>` to describe a type that accepts any javascript object.
+
+Sometimes a function or method may genuinely need to accept any object; eg:
+```js
+function encodeBody(body) {
+    // ...
+}
+```
+In this scenario:
+- Use `object` if you know that you will not access any property
+- Use `Record<string, any>` if you need to access some property
+
+Both usages prevent the type from accepting primitives (eg: string, boolean...). If using `Record`, ensure you have guards to check that the properties really do exist.