SOURCE

let obj = {
    "city" : "chicago", 
    "number": 20, 
    "user" : {
        "name":"Alex", 
        "age":20
        }
}

console.log(jsonschema(obj))

function jsonschema(obj){
    let res = {};
    res['type'] = typeof obj;
    if( typeof obj == 'object'){
        res['properties'] = {}
        for(let item in obj){
            res['properties'][item] = jsonschema(item)
        }
    }
    return res
}