JavaScript loop array: compare previous and next item

Loop array and compare the previous item and the next item.

My array:

const array = [{id: 1,name: 'A'},{id: 2,name: 'B'},{id: 3,name: 'B'},{id: 4,name: 'B'},{id: 5,name: 'C'},{id: 6,name: 'D'},{id: 7,name: 'E'},{id: 8,name: 'E'},{id: 9,name: 'E'}]

My code:

let result = []
array.forEach(function(item, index) {
    if (index > 0) {
        if (array[index].name == array[index - 1].name) {
            result.push(array[index - 1].name)
        }
    }
});

console.log('result :', result); // The result I got: [ 'B', 'B', 'E', 'E' ]

But I want this result:

let result = [ 'B', 'B', 'B', 'E', 'E', 'E' ]

In such cases, reduce is the key

 let result = array.reduce((acc, curr, index) => {
 const previousItem = array[index - 1];
 if (previousItem && curr.name === previousItem.name) {
      acc.push(curr.name);
 }
 return acc;
 }, []);

 console.log(result); // Output: [ 'B', 'B', 'B', 'E', 'E', 'E' ]

You just need to conditionally check the previous and next value.

const array = [{id: 1,name: 'A'}, {id: 2,name: 'B'}, {id: 3,name: 'B'}, {id: 4,name: 'B'}, {id: 5,name: 'C'}, {id: 6,name: 'D'}, {id: 7,name: 'E'}, {id: 8,name: 'E'}, {id: 9,name: 'E'}]
    let result = [];

    array.forEach(function(item, index) {

        let prev = index - 1; // Previous value
        let next = index + 1; // Next value

        if(index > 0) {
            if((array[prev].name == array[index].name) || (array[index].name == array[next].name)) {
                result.push(array[index].name)
            }
        }
    });

    console.log('result: ', result);

Use:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const array = [
{ id: 1, name: 'A' },
{ id: 2, name: 'B' },
{ id: 3, name: 'B' },
{ id: 4, name: 'B' },
{ id: 5, name: 'C' },
{ id: 6, name: 'D' },
{ id: 7, name: 'E' },
{ id: 8, name: 'E' },
{ id: 9, name: 'E' }
];
const result = [];
let prevName = null;
let count = 0;
array.forEach(item => {
if (item.name === prevName) {
count++;
} else {
if (count > 1) {
for (let i = 0; i < count; i++) {
result.push(prevName);
}
}
prevName = item.name;
count = 1;
}
});
if (count > 1) {
for (let i = 0; i < count; i++) {
result.push(prevName);
}
}
console.log('result:', result);

<!-- end snippet -->