What is type widening and how do you prevent it?

Question presented to candidate: "Why does let method = "GET" have type string but const method = "GET" has type "GET"? What is type widening, where does it bite you, and what are your options for stopping it?"

What a strong answer should cover:

• Widening is the compiler replacing a literal type ("GET", 3, true) with its general type (string, number, boolean) when the value lands in a mutable place
• Where it happens: let and var, object properties, array elements, inferred function return types, and copying a const into a let
• const with a literal initializer keeps the literal; a literal written with an explicit literal annotation is "non-widening" and survives a copy into a let
• The classic symptom: send(request.method) fails with "Argument of type 'string' is not assignable to parameter of type 'Method'"
• The fixes and their trade-offs: annotate the slot, as const (deep, readonly), as const on one property, satisfies, a literal return type, a primitive constraint such as T extends string or a const type parameter
• Widening is compile-time only; the run-time value is the same string either way

Code / implementation expected: Yes — an object literal passed to a function that accepts a literal union, the error, then two or three different fixes.