how to get scale type of d3 (require es6-shim)

ChartHelper.ts

class ChartHelper {
    public static checkScaleType<T, R>(scale: Scale<T, R>): ScaleType {
        const f = scale.copy.toString();
        switch (true) {
            case f.includes("d3_scale_linear")  : return "linear";
            case f.includes("d3_scale_identity"): return "identity";
            case f.includes("d3_scale_pow")     : return "power";
            case f.includes("d3_scale_log")     : return "log";
            case f.includes("d3_scale_quantize"): return "quantize";
            case f.includes("d3_scale_quantile"): return "quantile";
            case f.includes("d3_scale_ordinal") : return "ordinal";
            case f.includes("d3_time_scale")    : return "time";
        }
        throw new TypeError("unexpected type");
    }
}

Graph.d.ts

type ScaleType = "linear" | "identity" | "power" | "log" | "quantize" | "quantile" | "ordinal" | "time";
type Scale<T, R> =
    | d3.scale.Linear<T, R>
    | d3.scale.Identity
    | d3.scale.Pow<T, R>
    | d3.scale.Log<T, R>
    | d3.scale.Quantize<T>
    | d3.scale.Quantile<T>
    | d3.scale.Ordinal<T, R>
    | d3.time.Scale<T, R>
    ;