S-Expression to binary tree in JavaScript

binary_tree.js

function SExpression(nodes) {
    if (!/(\(|\))+/g.test(nodes)) {
        return 'E5'
    }
    let a = nodes.split(' ').map(i => i.replace(/\(|\)/g, '').split(','))
    let keyPair = a.map(i => i[0])
    let valuePair = a.map(i => i[1])
    let aE1 = keyPair.reduce((prev, curr) => {
        prev[curr] = (prev[curr] || 0) + 1
        return prev
    }, {})
    const E1 = Object.values(a).filter(e => e > 2)
    if (E1.length) {
        return 'E1'
    }
    
    let aE2 = (Array.from(new Set(a.map(e => JSON.stringify(e))))).map(e => JSON.parse(e))
    if (aE2.length < a.length) {
        return 'E2'
    }
    
    let aE4 = keyPair.filter(key => !valuePair.includes(key))
    if (aE4.length > 2) {
        return 'E4'
    } 
    
    let aE3 = Array.from(new Set(valuePair))
    if (aE3.length < valuePair.length) {
        return 'E3'
    }
    
    let root = aE4[0]
    let routes = getObj(a, root)
    
    return getNodes(routes, root)
}
function getNodes(obj, key) {
    return `(${key}${Object.keys(obj).reduce((prev, curr) => {
        if (Object.keys(obj[curr]).length) {
            prev += getNodes(obj[curr], curr)
        } else {
            prev += `(${curr})`
        }
        return prev
    }, ``)})`
}
function getObj(a, key) {
    return a.reduce((prev, curr) => {
        if (curr[0] === key) {
            prev[curr[1]] = getObj(a, curr[1])
        }
        return prev
    }, {})
}