gistfile1.js

function isNumeric(v) {
    return !isNaN(+v) // implicit typecasting to Number, taking advantage of JavaScript's type coercion
}

function checkForm() {
    var elms_to_test = ['coupon', 'ins', 'quantity', 'ref', 'dr']
      , el, i

    for (i = elms_to_test.length; i--; ) {
        // use getElementById to get a reference to the element with the given ID.
        el = document.getElementById(elms_to_test[i])
        // you don't need to test against an explicit boolean in javascript, any value considered truthy will do.
        // truthy values are any other than 0, false, undefined, null and empty string ("")
        if (!isNumeric(el.value)) { 
            alert('Not numeric')
            return false }}
}