Evaluate expression with -, + and *.

eva.js

const eva = (str) => add(str)

const add = str => str.split('+').reduce((total, exp) => {
  const s = exp.trim()
  const num = Number(s)
  return total + (isNaN(num) ? subtract(s) : num)
}, 0)

const subtract = str => str.split('-').reduce((total, exp, index) => {
  const s = exp.trim()
  const num = Number(s)
  const result = isNaN(num) ? multiply(s) : num

  return index ? total - result : result
}, 0)

const multiply = str => str.split('*').reduce((total, exp) => total * Number(exp.trim()), 1)

const check = fn => (param, expected) => {
  const result = fn(param)
  const pass = result === expected
  console.log('[testing]', param, '<expected>', expected, pass ? '√' : `x - ${result}`)
}

const print = check(eva)

print('1 + 2', 3)
print('1 - 2', -1)
print('2 * 3', 6)
print('0 * 3', 0)
print('-1 + 3', 2)
print('-1 - 4', -5)
print('-2 * 4', -8)
print('-2 * 4 + 3', -5)
print('-2 * 4+ 3', -5)
print('-2* 4+ 3', -5)
print('-2*4  +3', -5)
print('1 - 2 * 4', -7)
print('5 +2 * 4', 13)
print(' 4 - 2 * 4 + 3', -1)