El problema se puede resolver con Array.reduce() de la siguiente manera:

 const arr = [{ name: "first name", rolePosition: 85 }, { name: "second name", rolePosition: 91 }]; const result = arr.reduce((prev, curr) => prev.rolePosition > curr.rolePosition ? prev : curr , {}); console.log(result);

Esta es tu solución.

 const arr = [ { name: "first name", rolePosition: 85 }, { name: "second name", rolePosition: 91 } ]; const numbers = []; arr.forEach(el => numbers.push(el.rolePosition)); const max = Math.max(...numbers); console.log(max)

Puede usar Math.max , Array.prototype.find para crear una función similar _.maxBy lodash

 const maxBy = (arr, func) => { const max = Math.max(...arr.map(func)) return arr.find(item => func(item) === max) } maxBy([{ test: 1 }, { test: 2 }], o => o.test) // { test: 2 } // or use reduce const maxItem = arr.reduce(function(a, b) { return a.rolePosition >= b.rolePosition ? a : b }, {})