i have a function to calculate area of polygon, and here's my function

import {distance} from "mathjs"
function getArea(arrayCord) {
    let triangle =[]
    let area = 0.0
    let a =0.0
    let b =0.0
    let c =0.0
    let s =0.0
    for (let i = 0; i < arrayCord.length-2; i++) {
        a = distance(arrayCord[i],arrayCord[i+1])// p1-p2
        b = distance(arrayCord[i+1],arrayCord[i+2]) //p2 - p3
        c = distance(arrayCord[i],arrayCord[i+2]) //p3-p1
        s = (a+b+c)/2;
        triangle[i] = Math.sqrt(s*(s-a)*(s-b)*(s-c))
        area+= triangle[i]
    }
    return area;
}

when i copy the function into a typescript class and use it, i got this error that said

Operator '+' cannot be applied to types 'number | math.BigNumber' and 'number | math.BigNumber'

the function in my typescript was

          const getArea = (arryy:(number|undefined)[][]) =>{
            let triangle =[]
            let area = 0.0
            let a:(number|BigNumber) =0.0
            let b:(number|BigNumber) =0.0
            let c:(number|BigNumber) =0.0
            let s:(number|BigNumber) =0.0
            for (let i = 0; i < arryy.length-2; i++) {
              a = distance(arryy[i],arryy[i+1])// p1-p2
              b = distance(arryy[i+1],arryy[i+2]) //p2 - p3
              c = distance(arryy[i],arryy[i+2]) //p3-p1
              s = (a+b+c)/2;
              triangle[i] = Math.sqrt(s*(s-a)*(s-b)*(s-c))
              area+= triangle[i]
            }
            return area;
        }

how to solve this problem so that i can use my function on my typescript class?