A naïve approach at a SinglyLinkedList object in JS

singly-linked-list.js

class SinglyLinkedList {
  constructor(value = []) {
    if (Array.isArray(value)) {
      value.forEach(val => this.addAtEnd(val))
    } else {
      this.value = value
      this.next  = null
    }
  }

  find(target) {
    let res = this
    while (res) {
      if (res.value === target) return res
      res = res.next
    }
    return res
  }

  findBefore(target) {
    let res = this
    while (res.next) {
      if (res.next.value === target) return res
      res = res.next
    }
    return res
  }

  addAtBeginning(new_value) {
    const new_cell = new this.constructor(new_value)
    new_cell.next  = this.next
    this.next      = new_cell

    return this
  }

  addAtEnd(new_value) {
    const new_cell = new this.constructor(new_value)
    let last       = this
    while (last.next) {
      last = last.next
    }

    last.next = new_cell

    return this
  }

  insert(new_value) {
    const new_cell = new this.constructor(new_value)
    new_cell.next  = this.next
    this.next      = new_cell

    return this
  }

  delete(elem) {
    this.findBefore(elem.value).next = elem.next

    return this
  }

  [Symbol.iterator]() {
    let curr = this
    return {
      i: curr.value,
      next() {
        curr = curr.next
        if (curr) {
          return {
            value: curr.value,
            done: false
          }
        }

        return {
          value: undefined,
          done: true
        }
      }
    }
  }
}

const list = new SinglyLinkedList([4,5,6,7,3,2,1]);

console.log(list.find(5).value) // 5
console.log(list.findBefore(5).value) // 4
console.log(list.addAtBeginning(8).next.value) // 8

console.log(list.addAtEnd(10).find(10).value) // 10

const elem = list.find(5)
console.log(elem.insert(12).next.value) // 12

console.log(list.delete(elem).find(4).next.value) // 12

for (let cell of list) {
  console.log(cell) //8,4,12,6,7,3,2,1,10
}

It should probably be noted that this implementation utilizes a sentinel, so the first value of the list is not set. Makes managing the list way easier