chem.js

let get = str => {
    let amount = Number(str.match(/^\d*/)[0]) || 1;
    return { 
        comps: str.matchAll(/([A-Z][a-z]?)(\d?)/g)
            .toArray().map(([,c,a])=>({ c, a: Number(a) || 1 })),
        amount 
    };
}
let takes = args.join(" ");
let [left, right] = takes.replace(/\s/g, '')
    .split("->").map(e=>e.split("+"));

let [left_val, right_val] = [left, right].map(re => {
    let val = {};
    for (let thing of re) {
        let res = get(thing);
        res.comps.map(g => {
            val[g.c] ??= 0
            val[g.c] += res.amount * g.a
        })
    }
    return Object.fromEntries(Object.entries(val).sort((a, b) => a[0].charCodeAt() - b[0].charCodeAt()));
});
return ([left_val, right_val].map(e => JSON.stringify(e).replaceAll('"', '').replaceAll(/.(?=})|[{:,]/g, '$& ')).join(' -> '))