密碼強度，指一個密碼對抗猜測或是暴力破解的有效程度。設定一個高強度的密碼是非常重要的。

javascript核心程式碼

window.onload = function() {

			var PasswordInput = document.getElementById('password');
			PasswordInput.value = '';
			var sp = document.getElementsByTagName('span');

			PasswordInput.onkeyup = function() {
				//強度狀態設為預設
				sp[0].className = sp[1].className = sp[2].className = "default";

				var pwd = this.value;
				var result = 0;
				for(var i = 0, len = pwd.length; i < len; ++i) {
					result |= charType(pwd.charCodeAt(i));    
					//返回pwd第i位的Unicode編碼
				}

				var PasswordLevel = 0;
				//對result進行四次迴圈，計算其PasswordLevel
				for(var i = 0; i <= 4; i++) {
					if(result & 1) {
						PasswordLevel++;
					}
					//右移一位
					result = result >>> 1;
				}

				if(pwd.length >= 6) {
					//當輸入的密碼大於6位，進行判斷，switch判斷密碼強度並展示
					switch(PasswordLevel) {
						case 1:
							sp[0].className = "weak";
							break;
						case 2:
							sp[0].className = "medium";
							sp[1].className = "medium";
							break;
						case 3:
						case 4:
							sp[0].className = "strong";
							sp[1].className = "strong";
							sp[2].className = "strong";
							break;
					}
				}
			}
		}
		function charType(num) {
                //函式返回值返回在強度檢測switch裡面
			if(num >= 48 && num <= 57) {
				return 1;
			}
			if(num >= 97 && num <= 122) {
				return 2;
			}
			if(num >= 65 && num <= 90) {
				return 4;
			}
			return 8;
		}
 

html:

效果圖: