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'use strict';

// Projection

const projection = (metadata) => {

const meta = {};

let key;

for (const item of metadata) {

const type = typeof item;

let cast = projection[type];

if (type === 'string') key = item;

if (type === 'object') cast = cast(key);

meta[key] = cast(item);

}

const keys = Object.keys(meta);

const mapper = (obj) => {

const hash = {};

for (const key of keys) {

const cast = meta[key];

const value = cast(obj);

if (value) hash[key] = value;

}

return hash;

};

return mapper;

};

projection.string = (name) => (d) => d[name];

projection.function = (fn) => (d) => fn(d);

projection.object = (name) => (proj) => (d) => {

const data = d[name];

if (!data) return data;

return data.map(projection(proj));

};

// Dataset

const persons = [

{ name: 'Marcus Aurelius', city: 'Rome', born: 121, places: [

{ name: 'Shanghai', population: 24256800, country: 'China' },

{ name: 'Beijing', population: 21516000, country: 'China' },

{ name: 'Delhi', population: 16787941, country: 'India' }

] },

{ name: 'Victor Glushkov', city: 'Rostov on Don', born: 1923, places: [

{ name: 'Lagos', population: 16060303, country: 'Nigeria' },

{ name: 'Delhi', population: 16787941, country: 'India' },

{ name: 'Tianjin', population: 15200000, country: 'China' }

] },

{ name: 'Ibn Arabi', city: 'Murcia', born: 1165, places: [

{ name: 'Beijing', population: 21516000, country: 'China' },

] },

{ name: 'Mao Zedong', city: 'Shaoshan', born: 1893 },

{ name: 'Rene Descartes', city: 'La Haye en Touraine', born: 1596, places: [

{ name: 'Karachi', population: 14910352, country: 'Pakistan' },

{ name: 'Istanbul', population: 14160467, country: 'Turkey' },

{ name: 'Tianjin', population: 15200000, country: 'China' }

] },

];

// Metadata

const md = [

'name',

'place', (d) => `<${d.city.toUpperCase()}>`,

'born',

'age', (d) => (

new Date().getFullYear() -

new Date(d.born.toString()).getFullYear()

),

'places', [

'address', (d) => (d.country.toUpperCase() + ', ' + d.name),

'population'

]

];

// Usage

const p = projection(md);

const data = persons.map(p);

console.dir(data, { depth: 10 });