ps_decode_js.js

// Just a simple example to understand how JS object is decoded to purescript record.
// Skipping unwrapSingleArguments, unwrapSingleConstructors, sumEncoding.

purescript_obj = { "firstName" : "Saravana", "lastName" : "Murugesan"}
foreign_obj = { "first_name" : "Saravana", "last_name" : "Murugesan"}

const camelToSnakeCase = str => str.replace(/[A-Z]/g, letter => `_${letter.toLowerCase()}`);

function decode(foreign_obj , res,options){
    if (typeof res === 'object'){ // just a generalization
        decodeRecord(foreign_obj,res,options);
    }else{
        // encode primitves
        // Considering them as same
    }
    
    return res;
}
function decodeRecord(foreign_obj,res,options){
    // assume you know `purescript_obj` keys here
    Object.keys(purescript_obj).forEach(ps_key => {
          let js_key = options.fieldTransform(ps_key);
          
          if(foreign_obj.hasOwnProperty(js_key)){
              let js_val = foreign_obj[js_key];
              delete foreign_obj[js_key];
              res[ps_key] = decode(foreign_obj,js_val,options);
          }else{
              // if type is `Maybe` we can set the value as Nothing
              throw new Error(`Error At Property ${ps_key}` );
          } 
          
    });
}

// program starts here
res = {}
options = {fieldTransform : camelToSnakeCase}
decode(foreign_obj,res,options)
console.log(res)