You need to add EventListener() method

You can add an event listener.

  document.getElementById("fooId").addEventListener("change", func);
    function func(e){
     alert(e.target.value)
    }

<input type="text" id="fooId"/>

Update:

So user https://www.webdeveloper.com/u/Sempervivum helped me and found this script originaly posted by Shawn Regan here: detect value change in input tag with vanilla javascript and MutationObserver which does exactly what I was looking for.
You can find working demo here: https://jsfiddle.net/zyx54/hdn4txry/
That script here:

let registered = [];
let setDetectChangeHandler = function(field) {
  if (!registered.includes(field)) {
    let superProps = Object.getPrototypeOf(field);
    let superSet = Object.getOwnPropertyDescriptor(superProps, "value").set;
    let superGet = Object.getOwnPropertyDescriptor(superProps, "value").get;
    let newProps = {
      get: function() {
        return superGet.apply(this, arguments);
      },
      set: function (t) {
        let _this = this;
        setTimeout( function() { _this.dispatchEvent(new Event("change")); }, 50);
        return superSet.apply(this, arguments);
      }
    };
    Object.defineProperty(field, "value", newProps);
    registered.push(field);
  }
}

this should help.
https://www.w3schools.com/jsref/event_onchange.asp
else if you have more specific need, then share that

This way to add eventlistner to input.

var sampleFunction = function(values){
  document.getElementById("sample").value = "New text";
} 

var input = document.getElementById("sample");

input.addEventListener('change', (event) => { 

  console.log("afafaf");
});

<div>
 <input type="text" id="sample" onchange="sampleFunction(this.value)"/>
 
</div>

<span id="changetext" ></span>

When I use onChange(), it triggers only when user changes something in an input field. I want it to trigger even when JavaScript changes something in an input field. How can I achieve this?