Replace any object field with value empty string to null without iteration?

It's not possible to do without iteration. From an outside function's point of view, your object is simply an arbitrary set of key-value pairs.

That said, you could do it in a single line of code with a library like lodash and its mapValues function:

_.mapValues(myObj, v => v === '' ? null : v)

Or, you could do it yourself without an external library (this example mutates the original object):

Object.keys(myObj).forEach(k => myObj[k] = myObj[k] === '' ? null : myObj[k])

There's another way you can sort of do what you ask, using a Proxy object. This does what you're asking in a lazy sort of way. It won't actively change the values in your object, but when a value is requested, it'll check if it's an empty string and return null if it is:

const proxyObj = new Proxy(myObj, {
  get: (obj, prop) => obj[prop] === "" ? null : obj[prop],
});

// proxyObj.value1
//   => 'bacon'
// proxyObj.value2
//   => null

This Proxy solution may be clever, but the iterative solutions above are the simpler and better choice in most situations.

Another iteration example.

const obj = {
  value1: 'bacon',
  value2: '',
  value3: 'lard',
  value4: '',
  value5: 'cheese',
};

Object.keys(obj).reduce((acc, key) => {acc[key] = obj[key] === '' ? null : 
obj[key]; return acc; }, {})

For anyone who comes across this and these solutions don't work exactly like they want, here's how I solved it for myself. I used a JSON.stringify + JSON.parse(data, reviver) to translate the strings.

let data = {
  value1: 'bacon',
  value2: '',
  value3: 'lard',
  value4: '',
  value5: 'cheese',
};

data = JSON.parse(JSON.stringify(data), (key, value) => {
    return value === '' ? null : value;
});