1. class PriorityQueue {
    2.   constructor(
    3.     compare = (a, b) => a[0] < b[0] 
    4.     ){
    5.     this.data = []
    6.     this.size = 0
    7.     this.compare = compare
    8.   }
    10.   peek() {
    11.     return this.size === 0 ? null : this.data[0] 
    12.   }
    14.   offer(val) {
    15.     this.data.push(val)
    16.     this._shifUp(this.size++)
    17.   }
    19.   poll() {
    20.     if(this.size === 0) { return null }
    21.     this._swap(0, --this.size)
    22.     this._shifDown(0)
    23.     return this.data.pop()
    24.   }
    26.   _parent(index) {
    27.     return index - 1 >> 1
    28.   }
    30.   _child(index) {
    31.     return (index << 1) + 1
    32.   }
    34.   _shifDown(index) {
    35.     while(this._child(index) < this.size) {
    36.       let child = this._child(index)
    37.       if(child + 1 < this.size 
    38.         && this.compare(this.data[child + 1], this.data[child])) {
    39.           child = child + 1
    40.       }
    41.       if(this.compare(this.data[index], this.data[child])){
    42.         break
    43.       }
    44.       this._swap(index, child)
    45.       index = child
    46.     }
    47.   }
    49.   _shifUp(index) {
    50.     while(this._parent(index) >= 0 
    51.     && this.compare(this.data[index], this.data[this._parent(index)])) {
    52.       this._swap(index, this._parent(index))
    53.       index = this._parent(index)
    54.     }
    55.   }
    57.   _swap(a, b) {
    58.     [this.data[a], this.data[b]] = [this.data[b], this.data[a]]
    59.   }
    60. }