improved-readability.js

const makeCoordinatesZeroBased = rawCoordinates => Object.entries(rawCoordinates)
  .reduce((processedCoordinates, [coordinate, value]) => {
    const shouldConvert = typeof value === 'number';    
    const updatedValue = shouldConvert ? value - 1 : value;
    
    processedCoordinates[coordinate] = updatedValue;
    
    return processedCoordinates;
  }, {});