JSFiddle - React, Tailwind, and code Playground

JavaScript

m2a = a=>(b=[],c={},a.replace(/([A-Z][a-z]*)(?![0-9a-z])/g, "$11").match(/[A-Z][a-z]*|[0-9]+|[\[\(]/g).reverse().map(d=>(d*1==d&&b.push(d*1),d.match(/\(|\[/)&&b.pop(),d.match(/[A-Z]/)&&eval('e=b.reduce((f,g)=>f*g,1),c[d]=c[d]?c[d]+e:e,b.pop()'))),eval('g="";for(x in c)g+=x+`: ${c[x]}\n`'))

var tests = `
> Ba(PO3)2
Ba: 1
P: 2
O: 6

> C13H18O2
C: 13
H: 18
O: 2

> K4[ON[SO3]2]2
K: 4
O: 14
N: 2
S: 4

> (CH3)3COOC(CH3)3
C: 8
H: 18
O: 2

> [C2H5]2NH
C: 4
H: 11
N: 1

> Co3(Fe(CN)6)2
Co: 3
Fe: 2
C: 12
N: 12`;

tests.match(/\> (.*?)\n/g).map(item => (item = item.slice(2,-1),console.log(item, "\n", m2a(item))));