<button>test</button>

var h = [2, 4, 6, 3, 4, 2, 7];
var button = document.getElementsByTagName('button');
var placementOfValueReaches5 = [];

h.forEach(function(value, index){
    if (value >= 5){
        button[index].setAttribute('disabled', true);
        placementOfValueReaches5.push(index);
    }
});

console.log(placementOfValueReaches5);  // outputs [2, 6]