An example of using reduce in JavaScript to combine child arrays

reduce.js

/*
 * An example of using reduce to combine child arrays
 *
 * Expected Output:

    SOURCE:
    [
        {
            "name": "alpha",
            "kids": [
                "beta",
                "gamma"
            ]
        },
        {
            "name": "delta",
            "kids": [
                "epsilon",
                "hydra"
            ]
        }
    ]

    TARGET:
    [
        "beta",
        "gamma",
        "epsilon",
        "hydra"
    ]

 */

const source = [
    { name: 'alpha', kids: ['beta','gamma']},
    { name: 'delta', kids: ['epsilon','hydra']}
];

console.log(`SOURCE:\n ${JSON.stringify(source,null,2)}`);

/* 
 * You must end reduce with '[]' to tell JS that prev is an array
 * or you will get 'concat is not a function' (more like method of prev is not a function)
 */
const target = source.reduce( (prev, curr) =>  prev.concat(curr.kids), [] );

console.log(`TARGET:\n ${JSON.stringify(target,null,2)}`);