易混淆运算、易错知识点-《js高级程序设计》

break与continue

var num = 0;
    
outermost:
for (var i=0; i < 10; i++) {
     for (var j=0; j < 10; j++) {
        if (i == 5 && j == 5) {
            break outermost;
        }
        num++;
    }
}

alert(num);    //55

var num = 0;
        
for (var i=0; i < 10; i++) {
     for (var j=0; j < 10; j++) {
        if (i == 5 && j == 5) {
            break;
        }
        num++;
    }
}

alert(num);    //95

var num = 0;
        
outermost:
for (var i=0; i < 10; i++) {
     for (var j=0; j < 10; j++) {
        console.log("i-->"+i+" ,j-->"+j);
        if (i == 5 && j == 5) {
            continue outermost;  //  跳到外层 下次从i=6，j=0开始，所以少了5次
        }
        num++;
    }
}

alert(num);    //95

var num = 0;
        
for (var i=0; i < 10; i++) {
     for (var j=0; j < 10; j++) {
        console.log("i-->"+i+" ,j-->"+j);
        if (i == 5 && j == 5) {
            continue;  //跳出本次循环
        }
        num++;
    }
}

alert(num);    //99

除法

alert(5 / 5);           //1
alert(5 / NaN);         //NaN
alert(Infinity / Infinity);    //NaN
alert(Infinity / 2);    //Infinity
alert(5 / 0);           //Infinity
alert(10 / true);       //10
alert(10 / false);      //Infinity

等于 全等

alert(null == undefined);    //true
alert(null === undefined);   //false

alert("NaN" == NaN);        //false
alert("NaN" === NaN);       //false
alert(NaN == NaN);          //false
alert(NaN === NaN);         //false
alert(NaN != NaN);          //true
alert(NaN !== NaN);         //true

alert(false == 0);          //true
alert(false === 0);         //false
alert(true == 1);           //true
alert(true === 1);          //false

alert(null == 0);           //false
alert(undefined == 0);      //false

alert(5 == "5");            //true
alert(5 === "5");           //false      

arguments

 function doAdd(num1, num2) {
    //if(arguments.length == 1) {
        arguments[1] = 10;
    //}
    alert(arguments[0] + num2);
}

doAdd(10, 20);        //20
doAdd(30, 20);    //40

++ —

var s1 = "2";
var s2 = "z";
var b = false;
var f = 1.1;
var o = { 
    valueOf: function() {
        return -1;
    }
};

s1++;   //value becomes numeric 3
s2++;   //value becomes NaN
b++;    //value becomes numeric 1
f--;    //value becomes 0.10000000000000009
o--;    //value becomes numeric –2  

与或非 执行

var found = true;
var result = (found && someUndeclaredVariable);    //error occurs here
alert(result);    //this line never executes

var found = false;
var result = (found && someUndeclaredVariable);    //no error
alert(result);    //works

var found = true;
var result = (found || someUndeclaredVariable);    //no error
alert(result);    //works


var found = false;
var result = (found || someUndeclaredVariable);    //error occurs here
alert(result);    //this line never executes

alert(!false);      //true
alert(!"blue");     //false
alert(!0);          //true
alert(!NaN);        //true
alert(!"");         //true
alert(!12345);      //false


alert(!!"blue");     //true
alert(!!0);          //false
alert(!!NaN);        //false
alert(!!"");         //false
alert(!!12345);      //true

各种运算:取模、乘法、除法

alert(26 % 5);          //1
alert(Infinity % 3);    //NaN
alert(3 % 0);           //NaN
alert(5 % Infinity);    //5
alert(0 % 10);          //0
alert(true % 25);       //1
alert(3 % false);       //NaN


alert(5 * 6);           //30
alert(5 * NaN);         //NaN
alert(Infinity * 0);    //NaN
alert(Infinity * 2);    //Infinity
alert("5" * 5);         //25
alert(true * 10);       //10
alert(false * 10);      //0


alert(NaN == NaN);       //false
alert(isNaN(NaN));       //true
alert(isNaN(10));        //false – 10 is a number
alert(isNaN("10"));      //false – can be converted to number 10
alert(isNaN("blue"));    //true – cannot be converted to a number
alert(isNaN(true));      //false – can be converted to number 1


var num1 = Number("Hello world!");  //NaN
var num2 = Number("");              //0
var num3 = Number("000011");        //11
var num4 = Number(true);            //1


var num1 = parseInt("1234blue");    //1234
var num2 = parseInt("");            //NaN
var num3 = parseInt("0xA");         //10 - hexadecimal
var num4 = parseInt(22.5);          //22
var num5 = parseInt("70");          //70 - decimal
var num6 = parseInt("0xf");         //15 – hexadecimal

var num1 = parseInt("AF", 16);        //175
var num2 = parseInt("AF");            //NaN

var num1 = parseFloat("1234blue");    //1234 - integer
var num2 = parseFloat("0xA");         //0
var num3 = parseFloat("22.5");        //22.5
var num4 = parseFloat("22.34.5");     //22.34
var num5 = parseFloat("0908.5");      //908.5
var num6 = parseFloat("3.125e7");     //31250000


var result1 = 5 > 3;    //true
var result2 = 5 < 3;    //false
var result3 = "Brick" < "alphabet";  //true
var result4 = "Brick".toLowerCase() < "alphabet".toLowerCase();  //false
var result5 = "23" < "3";  //true
var result6 = "23" < 3;    //false
var result7 = "a" < 3;    //false because "a" becomes NaN
var result8 = NaN < 3;    //false
var result9 = NaN >= 3;   //false


var result1 = 5 - true;    //4 because true is converted to 1
var result2 = NaN - 1;     //NaN
var result3 = 5 - 3;       //2
var result4 = 5 - "";      //5 because "" is converted to 0
var result5 = 5 - "2";     //3 because "2" is converted to 2
var result6 = 5 - null;    //5 because null is converted to 0


var s1 = "01";
var s2 = "1.1";
var s3 = "z";
var b = false;
var f = 1.1;
var o = { 
    valueOf: function() {
        return -1;
    }
};

s1 = +s1;   //value becomes numeric 1
s2 = +s2;   //value becomes numeric 1.1
s3 = +s3;   //value becomes NaN
b = +b;     //value becomes numeric 0
f = +f;     //no change, still 1.1
o = +o;     //value becomes numeric –1


var s1 = "01";
var s2 = "1.1";
var s3 = "z";
var b = false;
var f = 1.1;
var o = { 
    valueOf: function() {
        return -1;
    }
};

s1 = -s1;   //value becomes numeric -1
s2 = -s2;   //value becomes numeric -1.1
s3 = -s3;   //value becomes NaN
b = -b;     //value becomes numeric 0
f = -f;     //change to –1.1
o = -o;     //value becomes numeric 1


var oldValue = -64;              //equal to binary 11111111111111111111111111000000
var newValue = oldValue >>> 5;   //equal to decimal 134217726
alert(newValue);                 //134217726