why is {[key: string]: T} not { x: number }

property.ts

interface Matrix {
  readonly [index: number]: {
    readonly [index: number]: number;
  };
}
interface CartesianPoint {
    x: number;
    y: number;
    z: number;
}

function generateMatrix_nPoints(...points: CartesianPoint[]): Matrix {
  return [
    [...points.map(property('x'))],
    [...points.map(point => point.y)],
    [...points.map(point => point.z)]
  ];
}

function property (name: string) {
  return function<T> (input: { [key: string]: T }): T {
    return input[name];
  }
}

You’ll have to defer the checking to runtime and cast it manually:

function generateMatrix_nPoints(...points: CartesianPoint[]): Matrix {
  return [
    [...points.map(property('x')) as number[]],
    [...points.map(point => point.y)],
    [...points.map(point => point.z)]
  ];
}

function property (name: string) {
    return function(input: object): any {
      if (!(name in input)) {
          throw new Error(`No ${name} in ${input}`);
      }

      return input[name];
  }
}