FenwickTree Javascript implementation

FenwickTree.js

class FenwickTree{
  constructor(array){
    this.source = [...array]
    this.length = this.source.length;
    this.fenwick = new Array(array.length+1).fill(0)
    this.build();
  }
  
  build(){
    for(let i = 0; i < this.length; i++)
      this.updateByDiff(i, this.source[i])
    
  }
  
  update(index, value){
    const diff = value - this.source[index];
    this.source[index] = value;
    this.updateByDiff(index, diff)
  }
  
  updateByDiff(index, diff){
    index += 1
    while(index < this.length){
      this.fenwick[index] += diff
      index += index & -index
    }
  }
  
  getSum(index){
    index += 1
    let ans = 0
    while(index > 0){
      ans += this.fenwick[index]
      index -= index & -index
    }
    return ans
  }
  
  getRangeSum(fromIndex, toIndex){
    return this.getSum(toIndex) - this.getSum(fromIndex-1)
  }
}

// example

const source = [1, 2, 3, 4, 5, 6,  9]

const bit = new FenwickTree(source);
console.log(bit.getRangeSum(2, 3))
bit.update(3, 9)
console.log(bit.getRangeSum(2, 3))