JSFiddle - React, Tailwind, and code Playground

JavaScript

function different_values(arr, value){
    var res = [];
    for(var i=arr.length;i--;){
        if(res.indexOf(arr[i][value]) === -1){
            res.push(arr[i][value])
        }
    }
    return res;
}

var arr =   [
      {
        "cod": 0,
        "level": "INF"
      },{
        "cod": 1,
        "level": "INF"
      },{
        "cod": 2,
        "level": "INC"
      },{
        "cod": 3,
        "level": "IND"
      }
]


alert(different_values(arr,'level'))