How to use the d3-scale-chromatic.interpolateSpectral function in d3-scale-chromatic

let set1: string = d3ScaleChromatic.schemeSet1[0]; // #e41a1c
let set2: string = d3ScaleChromatic.schemeSet2[0]; // #66c2a5
let set3: string = d3ScaleChromatic.schemeSet3[0]; // #8dd3c7

// -----------------------------------------------------------------------
// Diverging
// -----------------------------------------------------------------------
let BrBG: string = d3ScaleChromatic.interpolateBrBG(0); // rgb(84, 48, 5)
let PRGn: string = d3ScaleChromatic.interpolatePRGn(0); // rgb(64, 0, 75)
let PiYG: string = d3ScaleChromatic.interpolatePiYG(0); // rgb(142, 1, 82)
let PuOr: string = d3ScaleChromatic.interpolatePuOr(0); // rgb(127, 59, 8)
let RdBu: string = d3ScaleChromatic.interpolateRdBu(0); // rgb(103, 0, 31)
let RdGy: string = d3ScaleChromatic.interpolateRdGy(0); // rgb(103, 0, 31)
let RdYlBu: string = d3ScaleChromatic.interpolateRdYlBu(0); // rgb(103, 0, 31)
let RdYlGn: string = d3ScaleChromatic.interpolateRdYlGn(0); // rgb(103, 0, 31)
let Spectral: string = d3ScaleChromatic.interpolateSpectral(0); // rgb(158, 1, 66)

// -----------------------------------------------------------------------
// Sequential
// -----------------------------------------------------------------------
let Blue: string = d3ScaleChromatic.interpolateBlues(1); // rgb(8, 48, 107)
let Green: string = d3ScaleChromatic.interpolateGreens(1); // rgb(0, 68, 27)
let Grey: string = d3ScaleChromatic.interpolateGreys(1); // rgb(0, 0, 0)
let Orange: string = d3ScaleChromatic.interpolateOranges(1); // rgb(127, 39, 4)
let Purple: string = d3ScaleChromatic.interpolatePurples(1); // rgb(63, 0, 125)
let Red: string = d3ScaleChromatic.interpolateReds(1); // rgb(103, 0, 13)

// -----------------------------------------------------------------------
// Sequential(Multi-Hue)
// -----------------------------------------------------------------------
let BuGn: string = d3ScaleChromatic.interpolateBuGn(1); // rgb(0, 68, 27)
let BuPu: string = d3ScaleChromatic.interpolateBuPu(1); // rgb(77, 0, 75)

function _getColor(key: string, keys: string[], metadata: Metadata, scale) {
  if (metadata && metadata[key] && metadata[key].color) {
    return metadata[key].color;
  }
  return interpolateSpectral(scale(keys.indexOf(key)));
}