Check First Char In String

I have input-box. I'm looking for a way to fire-up alert() if first character of given string is equal to '/'...

var scream = $( '#screameria input' ).val();

if ( scream.charAt( 0 ) == '/' ) {

  alert( 'Boom!' );

}

It's my code at the moment. It doesn't work and I think that it's because that browser doesn't know when to check that string... I need that alert whenever user inputs '/' as first character.