Unique list of JavaScript objects.

uniqueByKey.ts

interface Item {
    type: string;
}

const listA: Item[] = [
    { type: 'a' },
    { type: 'b' },
    { type: 'c' },
];
const listB: Item[] = [
    { type: 'a' },
    { type: 'b' },
];
const listC: Item[] = [
    { type: 'b' },
    { type: 'c' },
    { type: 'd' },
];

const uniqueByKey = <T extends object, U extends keyof T>(key: U, ...items: T[][]) => {
    const flat = items.reduce((acc, next) => ([...acc, ...next]), []);
    const keys = [...new Set(flat.map(item => item[key]))];
    
    return keys.map(flatKey => flat.find(item => item[key] === flatKey));
}

console.log('unique objects:');
console.table(uniqueByKey('type', listA, listB, listC));