This check will reject code patterns that are likely to not be safe for an optimization known as “property renaming”. Closure compiler is currently one of the main JS optimizer that performs that optimization and anecdotally it results in 5-10% smaller bundles (see more info).
Property renaming is the optimization where writes and accesses to object properties like x.userName are rewritten to a shorter name like x.a. It works great until other code expects the property to be the specific string ‘userName’. This check will help catch bad optimization errors like this earlier (during TS compilation).
The check will reject a property access on an object through an index signature in its type. For example, if you write this:
interface IdxSig { [key: string]: string; }
function propAccess(x: IdxSig) { x.prop; // new error x[‘prop’]; // ok }
the compiler will start erring like this:
.../errors.ts:6:5 - error TS21227: Property prop is not declared on Type IdxSig. The type has a string index signature, but it is being accessed using a dotted property access.
6 x.prop; // new error
The presence of an index signature in the type most often means that the properties of the object that carries that type are created outside the static property renaming scope - for example JSON.parse, or dynamic for-in loop. Thus accessing them without a quoted index signature will result in an incorrect optimization.