JSFiddle - React, Tailwind, and code Playground

HTML

<label>TEXT 1: </label><input type="text" id="text_1" value=""/>
    <label>TEXT 2: </label><input type="text" id="text_2" value=""/>
    <label>TEXT 3: </label><input type="text" id="text_3" value=""/>
    <label>TEXT 4: </label><input type="text" id="text_4" value=""/>

JavaScript

/* INPUT TEXT_1 AND TEXT_2 VALUE TO TEXT_3 ON TEXT_1 KEYUP*/
$("#text_1").keyup(function() {
    $("#text_3").val($("#text_1").val() + " " + $("#text_2").val());
})

/* INPUT TEXT_1 AND TEXT_2 VALUE TO TEXT_3 ON TEXT_1 AND TEXT_2 KEYUP*/    
$("#text_2").keyup(function(){
    $("#text_3").val($("#text_1").val() + " " + $("#text_2").val());
})
    
/* HOW CAN I  DETECT IF TEXT_3 WAS CHANGED? IF TEXT_3 VALUE CHANGED, IT MUST BE INPUTTED TO TEXT_4*/

/* not working solution */    
$("#text_1, #text_2").keyup(function(){
    $("#text_4").val($("#text_3").val());
})