Javascript check if decimal of HTML form field is .0 or .5 -

i have javascript function fires on change of html form field adds decimal (.0) inputted number , checks value entered between 1 , 100. works fine need alter function check decimal inputted either .0 or .5 , refuse if not match. code:

     function wingsetdecimal(input) {               input.value = parsefloat(input.value).tofixed(1);               if (input.value <1 || input.value >100) {                   alert("wing length value above or below limits"); // accept value , not reset field // input.value ='';             }          }  

i don't know how approach issue check if decimal part .0 or .5. in other words field should reset input.value ='' if value inputted is, example 10.4 accepted if 10.0 or 10.5. have tried string.indexof(".")==-1; no success.

just 2 cents, perhaps trick:

if ((input.value % 1) * 10 === 0 || (input.value % 1) * 10 === 5) {     // logic comes 'ere } 

or derek has pointed out in comments, works in ever shorter form:

if(!(input.value % 0.5)) {     // todo }}