Requesting help with Javascript

I am learning Javascript. What is wrong with the following code?

var str = prompt("Enter some text");
var numChars = str.length;
for (var i = 0; i < numChars; i++)
{
if (str.slice(i, i + 2) == " ")
{
alert("No double spaces!");
break;
}
else {
alert("There is double space");
break;
}