javascript tricks - array and objects

js-tricks.js

[...Array(10).keys()]
// [1, 2, 3, 4, 5, 6, 7, 8, 9]

[...Array({ one:'1' }, { two: '2' }, { three: '3' }).keys()] 
// [1, 2, 3]


let arr1 = [1, 2, 3]
let arr2 = [2, 3, 4]

arr1.filter(i => !arr2.includes(i))
// [1]


let arr3 = [1, 2, 3]
let arr4 = [2, 3, 4]

arr3.filter(value => arr4.indexOf(value) !== -1)
// [2, 3]


let arr5 = [
    { name: "Peter", city: "New York" },
    { name: "Jane", city: "Berlin" },
    { name: "John", city: "Berlin" }
];

arr5.reduce((res, obj) => {
    res[obj.city] = res[obj.city] || []
    res[obj.city].push(obj)
    return res
}, {})

// {
//   "New York": [{ name: "Peter", city: "New York" }],
//   "Berlin":   [{ name: "Jane", city: "Berlin" }, { name: "John", city: "Berlin" }]
// }


let obj = {
    key1: "values1",
    key2: "values2"
}

Object.entries(obj)
// [["key1", "value1"], ["key2", "value2"]]


let arr6 = [{ prop: 1}, { prop: 3}, { prop: 2}]

arr6.reduce((res, obj) => res.prop > obj.prop ? res : obj)
// 3
arr6.reduce((res, obj) => res.prop < obj.prop ? res : obj)
// 1


let obj1 = { prop1: "prop1" };
let obj2 = { prop2: "prop2" };
let merged1 = { ...obj1, ...obj2};
// or
let merged2 = Object.assign(obj1, obj2);

// { prop1: "prop1", prop2: "prop2" }


"hello".split("").map(_ => "x").join("");
// "xxxxx"


let obj = { prop: "prop" }
Object.keys(obj).map((key, i) => obj[key] + i)
// ['prop0']