index.html

<input type="text" name="telefone" id="telefone" maxlength="15" />

telefone.js

function mascara(o, f) {
    v_obj = o;
    v_fun = f;
    setTimeout(execmascara, 1);
}

function execmascara() {
    v_obj.value = v_fun(v_obj.value)
}

function mtel(v) {
    v=v.replace(/\D/g,"");
    v=v.replace(/^(\d{2})(\d)/g,"($1) $2");
    v=v.replace(/(\d)(\d{4})$/,"$1-$2");
    return v;
}

function id(el) {
  return document.getElementById(el);
}

window.onload = function() {
	var $telefone = id('telefone');
	$telefone.onkeyup = function() {
		mascara( this, mtel );
	}
	$telefone.onblur = function() {
		var v = this.value;
		if( v.indexOf('(11)') !== -1 && v.length === 14) {
			this.value = v.replace(/\((\d{2})\) (\d{4})-(\d{4})/g,'($1) 9$2-$3');
		}
	}
}
 

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