jquery限制文本框输入字符长度

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    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);