You don't really need to do this in JS, just add a "maxlength" attribute:
<input type="text" maxlength="70"> or
The maxlength attribute for
<input type="text"> is supported across all modern browsers.
However the maxlength attribute is new for the
<textarea> tag in HTML5 and is not supported in Internet Explorer 9 (and earlier versions), or in Opera.
Also, please be aware that it would be relatively trivial to circumvent any client-side restrictions you place on input length and it is best to check again on the server, that the input matches the desired format.
If you need any more help with this, just let us know.