I often needed (and I think others do) to get type of element of collection, especially when dealing with collections of complex type. As a workaround I introduce new dummy variable which is assigned with a value of collection element in order to get its type further through the typeof. Here is a sample with simple collection of type number[]

function test(a: number[]) {
    var dummy = a[0]

    return a.map(mapper)

    function mapper(v: typeof dummy) {
        // some code
    }
}

A bad things about this approach is need to introduce new dummy variable, and some unclarified declarations function mapper(v: typeof dummy)

So my proposal is to provide some kind of metadata (which is common for C++ STL, boost) in form of exported type properties like <...container type...>.ElementType.

Here is not a final syntax, but at least it looks more clear

function test(a: number[]) {
    return a.map(mapper)

    function mapper(v: (typeof a).ElementType) {
        // some code
    }
}