jquery限制文本框输入字符长度

js代码：

jQuery.fn.maxLength = function(max){
        this.each(function(){
            var type = this.tagName.toLowerCase();
            var inputType = this.type? this.type.toLowerCase() : null;
            if(type == "input" && inputType == "text" || inputType == "password"){
                //Apply the standard maxLength
                this.maxLength = max;
            }
            else if(type == "textarea"){
                this.onkeypress = function(e){
                    var ob = e || event;
                    var keyCode = ob.keyCode;
                    var hasSelection = document.selection? document.selection.createRange().text.length > 0 : this.selectionStart != this.selectionEnd;
                    return !(this.value.length >= max && (keyCode > 50 || keyCode == 32 || keyCode == 0 || keyCode == 13) && !ob.ctrlKey && !ob.altKey && !hasSelection);
                };
                this.onkeyup = function(){
                    if(this.value.length > max){
                        this.value = this.value.substring(0,max);
                    }
                };
            }
        });
    };

使用方法：

$('#phone').maxLength(4);