You first need to parse the JSON into an suitable JS structure. Because of the nested nature of the data you need to 1) map over the first object in your array, 2) return the value of the value property of the first object of each and finally 3) check to see if the keyword is included in the the returned array, and return true or false.

const json = '[{\"type\":\"A_PRODUCT\",\"value\":[[{\"key\":\"SUBCLASS\",\"value\":\"1574\"}],[{\"key\":\"SUBCLASS\",\"value\":\"2331\"}]]}]';

const data = JSON.parse(json);

function doesItExist(data, keyword) {
  const values = data[0].value.map(arr => arr[0].value);
  return values.includes(keyword);
}

console.log(doesItExist(data, '2331'));
console.log(doesItExist(data, 'Bob'));
console.log(doesItExist(data, '1574'));

I will do something like this to loop

var a  = "[{\"type\":\"A_PRODUCT\",\"value\":[[{\"key\":\"SUBCLASS\",\"value\":\"1574\"}],[{\"key\":\"SUBCLASS\",\"value\":\"2331\"}]]}]";
var new_a = JSON.parse(a);
var value_compare = '1574';
new_a[0]['value'].forEach(element => {
    if (element[0].value == value_compare) {
        //DO SOMETHING
        alert('found: '+ JSON.stringify(element));
    }
});