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display entire array after moving values of it to another array

I have an array containing objects where there is a rpId key in some of the objects. The goal is to separate/move the objects that return undefined to a separate array and remove them out of the first array.
e.g.:
results = [{id: 1}, {id: 2, rpId: 1076}, {id: 3}, {id: 4, rpId: 303}];
goal: results = [{id: 2, rpId: 1076}, {id: 4, rpId: 303}] and stations = [{id: 1}, {id: 3}]
My current approach can be seen below. As visible, I get a wrong array1 because it contains an object with a rpId, plus array2 returns the keys of the object and I'd like to read the entire object, not just the "undefined" of the key.
const array1 = [{id: 1}, {id: 2, rpId: 1076}, {id: 3}, {id: 4, rpId: 303}];
const array2 = [];
const mapping = array1.map((e) => e.rpId);
console.log("mapping",mapping);
mapping.forEach(function(elem, index){
elem === undefined ? array2.push(elem) && array1.splice(index, elem === undefined) && console.log(elem): console.log("defined", elem);
}),
console.log("1", array1); // [{ id: 2, rpId: 1076 }, { id: 3 }]
console.log("2", array2); // [undefined, undefined]

Just check if the rpId property is undefined in each element.
const array1 = [{id: 1}, {id: 2, rpId: 1076}, {id: 3}, {id: 4, rpId: 303}];
const array2 = [];
array1.forEach(function(elem, index){
if(elem.rpId === undefined)
array2.push(elem) && array1.splice(index, 1)
});
console.log(array1);
console.log(array2);
One can also use Array#filter or push elements into two separate arrays based on the condition for better performance.
const array1 = [{id: 1}, {id: 2, rpId: 1076}, {id: 3}, {id: 4, rpId: 303}];
const yes = [], no = [];
array1.forEach(elem=>(elem.rpId!==undefined?yes:no).push(elem));
console.log(yes);
console.log(no);

You can use filter too:
let results = [
{ id: 1 },
{ id: 2, rpId: 1076 },
{ id: 3 },
{ id: 4, rpId: 303 },
];
const stations = results.filter((c) => !c.rpId);
results = results.filter((c) => c.rpId);
console.log("stations", stations);
console.log("results", results);

const GiveItACreativeName = (arr) => {
const result = []
const stations = []
arr.forEach((el) => {
if('rpId' in el) result.push(el);
else stations.push(el);
});
return {result, stations}
}
console.log(
GiveItACreativeName([{id: 1}, {id: 2, rpId: 1076}, {id: 3}, {id: 4, rpId: 303}])
);

Convert array of objects in an array of array of objects [duplicate]

This question already has answers here:
How can I group an array of objects by key?
(32 answers)
Closed 2 years ago.
I am retrieving data from a football (soccer) API. The specific data I need is an array of objects (306 objects). Every object has a property called matchday with a numeric value. I want to group all the objects that share the same property and store them in an array. What I need in the end is an array of array of objects.
Example array of objects:
[
{id: 264796, matchday: 1, …},
{id: 264797, matchday: 1, …},
{id: 264798, matchday: 2, …},
{id: 264800, matchday: 2, …},
]
What I want looks like this:
[
[{id: 264796, matchday: 1, …},{id: 264797, matchday: 1, …}],
[{id: 264798, matchday: 2, …},{id: 264800, matchday: 2, …}],
]

You can use .reduce() with Object.values() to get the desired output:
const data = [
{id: 264796, matchday: 1}, {id: 264797, matchday: 1},
{id: 264798, matchday: 2}, {id: 264800, matchday: 2}
];
const result = Object.values(
data.reduce((r, c) => {
r[c.matchday] = r[c.matchday] || [];
r[c.matchday].push(c);
return r;
}, {})
);
console.log(result);
.as-console-wrapper { max-height: 100% !important; top: 0; }

we can use reduce
const arr = [
{id: 264796, matchday: 1},
{id: 264797, matchday: 1},
{id: 264798, matchday: 2},
{id: 264800, matchday: 2},
]
const result = arr.reduce((acc, item) => {
if (!acc.find(accSubArr => accSubArr.find(accSubArrItem => accSubArrItem.matchday === item.matchday))) {
acc.push(arr.filter(arrItem => arrItem.matchday === item.matchday))
}
return acc;
}, [])
console.log(result)

You can try this:
const data = [{
id: 264796,
matchday: 1
},
{
id: 264797,
matchday: 1
},
{
id: 264798,
matchday: 2
},
{
id: 264800,
matchday: 2
},
]
const group = data
.map(d => d.matchday)
.filter((v, i, c) => c.indexOf(v) === i)
.map(i => data.filter(d => d.matchday === i))
console.log(group)

To add to the existing answers, here's another way making use of Map with a predicate to determine the group-by value:
const groupBy = predicate => items =>
Array.from(items.reduce((agg, next) => {
const key = predicate(next);
return agg.set(key, [].concat(agg.get(key) || []).concat(next));
}, new Map()).values());
const data = [
{name: 'A', key: 1},
{name: 'B', key: 1},
{name: 'C', key: 2},
{name: 'D', key: 2},
{name: 'E', key: 3}
];
const grouped = groupBy(x => x.key)(data);
For the fun of it, here's a recursive version of groupBy:
const groupBy = predicate => function group([next, ...items], grouped = new Map()) {
if (!next) {
return Array.from(grouped.values())
}
const key = predicate(next);
return group(items, grouped.set(key, [...(grouped.get(key) || []), next]));
}
And what the heck, here's a more imperative approach to add to the mix:
const groupBy = predicate => items => {
const cache = {};
for(item of items) {
const key = predicate(item);
cache[key] = [].concat(cache[key]).concat(item).filter(x => x)
}
return Object.values(cache);
}

Get maximum values of property from array of objects by id [duplicate]

This question already has answers here:
JavaScript "new Array(n)" and "Array.prototype.map" weirdness
(14 answers)
Closed 4 years ago.
I have an array of objects that looks like this:
const input = [
{id: 3, value: 2},
{id: 0, value: 3},
{id: 2, value: 8},
{id: 1, value: 5},
{id: 0, value: 2},
{id: 1, value: 6}
]
And I am trying to build an array of the maximum value by id, with id as index. For our example the desired output is the following:
const output = [ 3, 6, 8, 2 ]
I am also assuming that I know the number of unique ids ahead of time, and that they start at 0 and rise sequentially. My first whack at this was to .map() over the an empty array of the right length and build intermediate arrays with .filter() for each id, then use Math.max() on the filtered arrays.
const myAttempt = Array(4).map((_, index) => {
const filtered = input.filter(item => item.id === index);
return Math.max(filtered);
});
All I get out of this is:
myAttempt = [ 4 empty slots ];
I suspect I'm totally off-base with the Array(4) bit, and that the answer might involve .reduce(), but I never really got the hang of reduce so any help would be very much appreciated.
PS: I'd prefer answers that avoid the use of libraries in the vein of lodash or jQuery.

Use Array.reduce() to collect the values highest value of each key. Convert to array using Array.values():
const input = [
{id: 3, value: 2},
{id: 0, value: 3},
{id: 2, value: 8},
{id: 1, value: 5},
{id: 0, value: 2},
{id: 1, value: 6}
]
const result = Object.values(input.reduce((r, { id, value }) => {
r[id] = +r[id] > value ? r[id] : value;
return r;
}, {}));
console.log(result);
If all ids from 0 on wards appear in the array, you can add the values by their id (index) to an array accumulator:
const input = [
{id: 3, value: 2},
{id: 0, value: 3},
{id: 2, value: 8},
{id: 1, value: 5},
{id: 0, value: 2},
{id: 1, value: 6}
]
const result = input.reduce((r, { id, value }) => {
r[id] = +r[id] > value ? r[id] : value;
return r;
}, []);
console.log(result);

How to remove second square brackets from an array using JavaScript?

I have an array something like this:
data: [
[
{}, {} ... //multiple objects
]
]
How do I remove those second square brackets? I want it to be changed from [[{}]] to [{}].

Extract first item of your array
var data = [
[
{id: 1},
{id: 2}
]
]
console.log(data);
var newData = data[0];
console.log(newData);

You can use Array.flat() to flatten sub-arrays:
const data = [
[
{id: 1},
{id: 2}
],
[
{id: 3},
{id: 4}
]
];
const newData = data.flat();
console.log(newData);
If Array.flat() is not supported, you can spread the array into Array.concat():
const data = [
[
{id: 1},
{id: 2}
],
[
{id: 3},
{id: 4}
]
];
const newData = [].concat(...data);
console.log(newData);

Javascript concat and overwrite where element has the same id

Is it possible to concat two arrays with objects and let the second array overwrite the first array where they have the same id:
// array 1
[
{id: 1, name: "foo"},
{id: 2, name: "bar"},
{id: 3, name: "baz"}
]
// array 2:
[
{id: 1, name: "newFoo"},
{id: 4, name: "y"},
{id: 5, name: "z"}
]
// out:
[
{id: 1, name: "newFoo"}, // overwriten by array 2
{id: 2, name: "bar"}, // not changed (from array 1)
{id: 3, name: "baz"}, // not changed (from array 1)
{id: 4, name: "y"}, // added (from array 2)
{id: 5, name: "z"} // added (from array 2)
]
If it is possible I would like to do this without the use of third party libraries

var a = [
{id: 1, name: "foo"},
{id: 2, name: "bar"},
{id: 3, name: "baz"}
];
var b = [
{id: 1, name: "fooboo"},
{id: 4, name: "bar"},
{id: 5, name: "baz"}
];
/* iterate through each of b, if match found in a, extend with that of a. else push into b ...*/
b.forEach(m => {
var item = a.find(n => n.id === m.id);
if(item) { return Object.assign(item, m); }
a.push(m);
});
console.log(a);

You can do
let arr1 = [
{id: 1, name: "foo"},
{id: 2, name: "bar"},
{id: 3, name: "baz"}
]
let arr2 = [
{id: 1, name: "newFoo"},
{id: 4, name: "y"},
{id: 5, name: "z"}
]
let result = arr1.concat(arr2).reduce((a, b) => {
a[b.id] = b.name;
return a;
},{})
result = Object.keys(result).map(e => {
return {id : e, name : result[e]};
});
console.log(result);
Explanation
I am using the property of objects that they don't keep duplicate keys, so for an array concated together, I reduce it to an object with id as it's key and name as its value, hence overriding all duplicates. In the next step I converted this back into an array.

Check you my solution. There is no "rewrite", i just use a second array as base and don't write value if it has same id.
let a = [
{id: 1, name: "foo"},
{id: 2, name: "bar"},
{id: 3, name: "baz"}
];
let b = [
{id: 1, name: "newFoo"},
{id: 4, name: "y"},
{id: 5, name: "z"}
];
let duplicateId;
a.forEach(aitem => {
duplicateId = false;
b.forEach(bitem => {
if (aitem.id === bitem.id)
duplicateId = true;
});
if (!duplicateId)
b.push(aitem);
});
console.log(b);

Maybe you can use Object.assign and Object.entries to achieve, lets say:
const arr1 = [
{id: 1, name: "foo"},
{id: 2, name: "bar"},
{id: 3, name: "baz"}
]
const arr2 = [
{id: 1, name: "newFoo"},
{id: 4, name: "y"},
{id: 5, name: "z"}
]
const obj3 = Object.entries(Object.assign({}, ...arr1, arr2))
.map(([prop, value]) => ({[prop]:value}));
Example:
https://jsfiddle.net/0f75vLka/

Another option would be to convert arrays to map with id as key then merge the objects and then convert it back to array.
var arr1 = [
{id: 1, name: "foo"},
{id: 2, name: "bar"},
{id: 3, name: "baz"}
];
var arr2 = [
{id: 1, name: "newFoo"},
{id: 4, name: "y"},
{id: 5, name: "z"}
];
function arr2map(arr) {
var map = {};
for (var i = 0; i < arr.length; i++) {
var item = arr[i];
map[item.id] = item;
}
return map;
}
function map2arr(map) {
var arr = [];
for (var i in map) {
arr.push(map[i]);
}
return arr;
}
var arr1m = arr2map(arr1);
var arr2m = arr2map(arr2);
var arr3m = map2arr( Object.assign({}, arr1m, arr2m) );
//output
alert(JSON.stringify(arr3m));