formatKey.js

let isRealNumber = key => /^[0-9]+$/.test(key)
let isOKforDotNotation = key =>  /^[a-zA-Z_][a-zA-Z0-9_]*$/.test(key)

function wrapKey(key){
    if(isOKforDotNotation(key)){
        return `.${key}`
    } else if(isRealNumber(key)){
        return `[${key}]`
    } else {
       return `["${key}"]`
    }
}

module.exports = function formatKey(pathArray){
    return pathArray.reduce((prev, curr) => 
        prev + wrapKey(curr)
    )
}