JSFiddle - React, Tailwind, and code Playground

JavaScript

var obj = {
    a: [1,2,3],
    b: [4,5,6],
    c: [7,8,9]
}

var arr = obj.a.concat(obj.b);

console.log(arr)

var arr = [];
for (var k in obj) {
  arr.push.apply(arr, obj[k]);
}

console.log(arr)

var arr = [];
for (var k in obj) {
  arr = arr.concat(obj[k]);
}

console.log(arr)