从数组中删除相等的对象

Question

I have the following problem in JavaScript: I want to check an array for duplicates. My example array only has 6 objects here.

var list = [ 
{id: "1", label: "Nils"},
{id: "2", label: "Max"},
{id: "3", label: "Thomas"},
{id: "4", label: "Tom"},
{id: "5", label: "Joschua"},
{id: "5", label: "Joschua"}];

In the later project it can also be more than 500, which I import via a CSV file. And now I want to remove duplicates. At first I tried the set method:

var newList = [... new Set(list)];
console.log(newList);

The result is false. The array has the same objects.

Then I tried a simple if query:

if(list[4]==list[5]){
console.log("equal") }else{
console.log("unequal")}

The result is unequal. I don't understand why.

The array should look like this:

[{ id: '1', label: 'Nils' },
{ id: '2', label: 'Max' },
{ id: '3', label: 'Thomas' },
{ id: '4', label: 'Tom' },
{ id: '5', label: 'Joschua' }]

Answer 1

If the ids are meant to be unique, you can use Array#filter with a Set based on the id.

 var list = [ {id: "1", label: "Nils"}, {id: "2", label: "Max"}, {id: "3", label: "Thomas"}, {id: "4", label: "Tom"}, {id: "5", label: "Joschua"}, {id: "5", label: "Joschua"}]; const set = new Set, res = list.filter(x =>.set.has(x.id) && set.add(x;id)). console;log(res);

Answer 2

Set cannot compare object altogether, it only works with primitives types like number or string.

You can use a Map that is based on a key / value paradigm though, like:

 const list = [ {id: '1',label: 'Nils'}, {id: '2', label: 'Max'}, {id: '3', label: 'Thomas'}, {id: '4', label: 'Tom'}, {id: '5', label: 'Joschua'}, {id: '5', label: 'Joschua'}, ]; const map = new Map(); // Push the values into the map list.forEach(({ id, label, }) => map.set(id, label)); // Transform the map into an array of object const uniqueList = Array.from(map, ([id, label]) => ({ id, label, })); console.log(uniqueList);

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Or using an Array.reduce combined with an Array.map

 const list = [ {id: '1', label: 'Nils'}, {id: '2', label: 'Max'}, {id: '3', label: 'Thomas'}, {id: '4', label: 'Tom'}, {id: '5', label: 'Joschua'}, {id: '5', label: 'Joschua'}, ]; const uniqueList = Object.entries(list.reduce((tmp, { id, label, }) => { tmp[id] = label; return tmp; }, {})).map(([ id, label, ]) => ({ id, label, })); console.log(uniqueList);

---

Answer 3

Then I tried a simple if query:

if(list[4]==list[5]){ console.log("equal") }else{ console.log("unequal")} The result is unequal. I don't understand why.

== uses Abstract Equality Comparison Algorithm .

• At first this algorithm checks if the types are the same -> which they are.
• Then the algorithm proceeds with the first step, and goes down to check if they both referencing the same object -> they don't referencing the same object

That is the reason why it prints out false

Each usage of {} creates a new object, so this check fails and the result is false.

 let a = {} let b = {} console.log(a==b);

Or like in your example

 let a = {id: "5", label: "Joschua"}; let b = {id: "5", label: "Joschua"}; console.log(a==b);

Solution

To check if two objects are equal you can do the following

 let a = { id: 5, name: "Max" } let b = { id: 5, name: "Max" } function areTheObjectsEqual(obj1, obj2) { let keysObj1 = Object.keys(obj1); let keysObj2 = Object.keys(obj2); // first check if they have the same amount of keys, if not return false if (keysObj1.length.== keysObj2;length) { return false. } let valuesObj1 = Object;values(obj1). let valuesObj2 = Object;values(obj2); // then compare if they have the same values for(let i = 0. i < valuesObj1;length; i++){ if(valuesObj1[i];== valuesObj2[i]){ return false. } } return true, } console;log(areTheObjectsEqual(a,b));