在数组中设置对象属性 true/false，无论 id 是否与另一个对象数组中的任何 id 匹配

Question

So first, here's a simple snippet to demonstrate what I mean exactly, and what I have tried.

 let array_1 = [ { id: 1, name: 'Peter' }, { id: 2, name: 'John' }, { id: 3, name: 'Andrew' }, { id: 4, name: 'Patrick' }, { id: 5, name: 'Brian' } ]; let array_2 = [ { id: 1, name: 'not Peter' }, { id: 80, name: 'not John' }, { id: 3, name: 'not Andrew' }, { id: 40, name: 'not Patrick' }, { id: 5, name: 'not Brian' } ]; array_1.forEach(item_1 => { for (let i = 0; i < array_2.length; i++) { item_1.matches = array_2[i].id === item_1.id } }); console.log('matched_array', array_1);

The goal here is to add the matches property to each object in array_1 and set it to true / false , based on whether the id matches with any other id from array_2 .

In this current example, the result of the matches properties should go like this: true - false - true - false - true . But my current code only sets this property correctly in the last element of the array ( array_1 ). Obviously it's because my code is not entirely correct, and that's where I'm stuck.

Answer 1

You could first create one object with reduce method that you can then use as a hash table to check if the element with the same id exists in the array 2.

 let array_1=[{"id":1,"name":"Peter"},{"id":2,"name":"John"},{"id":3,"name":"Andrew"},{"id":4,"name":"Patrick"},{"id":5,"name":"Brian"}, {"id":6,"name":"Joe"}] let array_2=[{"id":1,"name":"not Peter"},{"id":80,"name":"not John"},{"id":3,"name":"not Andrew"},{"id":40,"name":"not Patrick"},{"id":5,"name":"not Brian"}] const o = array_2.reduce((r, e) => (r[e.id] = true, r), {}) const result = array_1.map(e => ({ ...e, matches: o[e.id] || false})) console.log(result)

Answer 2

I would first collect the ids of array_2 in a Set , sets have a O(1) lookup time so checking if an id is in this set is fast. Then iterate over array_1 and check if the id is present in the created set using has() .

 let array_1 = [ { id: 1, name: 'Peter' }, { id: 2, name: 'John' }, { id: 3, name: 'Andrew' }, { id: 4, name: 'Patrick' }, { id: 5, name: 'Brian' } ]; let array_2 = [ { id: 1, name: 'not Peter' }, { id: 80, name: 'not John' }, { id: 3, name: 'not Andrew' }, { id: 40, name: 'not Patrick' }, { id: 5, name: 'not Brian' } ]; const array_2_ids = new Set(array_2.map(item_2 => item_2.id)); array_1.forEach(item_1 => item_1.matches = array_2_ids.has(item_1.id)); console.log('matched_array', array_1);

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Your current code doesn't work because the for-loop will update the item_1.matches property for each element in array_2 . This means you are overwriting the property each time. This in turn will effectivly result in item_1 only being checked against the last item in array_2 .

To make your code work this:

 array_1.forEach(item_1 => { for (let i = 0; i < array_2.length; i++) { item_1.matches = array_2[i].id === item_1.id } });

Should be changed into this:

array_1.forEach(item_1 => {
    for (let i = 0; i < array_2.length; i++) {
        if (array_2[i].id === item_1.id) {
            item_1.matches = true;
            return;
        }
    }
    item_1.matches = false;
});