Test Record

来源：互联网发布：淘宝直播好申请吗编辑：程序博客网时间：2024/06/06 23:16

1. Type

10 + 20/0//Infinity'10' + 20/''//10Infinity

2. Variate

(1) not defined

var foo = "hello";(function(){    var bar = " world";    alert(foo + bar);})();alert(foo + bar);//-hello world //-error: bar is not defined

如果var 了bar，但没有赋值，才是 undefined。
没有var 就是 not defined。

(2) Are the following statements equivalent, if not, what’s the difference

var a = b = c = 1;var c = 1, b = c, a = b;//No, the former one creates global variables b and c.

3. SetTimeout

(function () {    console.log(1);    setTimeout(function () {        console.log(2);    }, 100);    console.log(3);    setTimeout(function () {        console.log(4);    }, 0);    console.log(5);})()//1, 3, 5, 4, 2

4. Join

（function () {    var a = [1];    function bar() {        if (!a) {            var a = [1, 2];        }        console.log(a.join());    }    bar();})()//1, 2

没有块级作用域，只有函数作用域。作用域中变量声明会前置。

5. Write the ‘add’ function that makes the following statement true:

add(3,4) === add(3)(4)

function add() {  var arg1 = arguments [0];  return arguments.length == 2 ? arg1 + arguments[1] : function (arg2) {    return arg1 + arg2;  }}

如何理解这样的代码？我们做一个翻译

function add(){    var value;    var arg1 = arguments[0];    if(arguments.length==2){        value = arg1+arguments[1];    }else{        value = function(arg2){            return arg1+arg2;        }    }    return value;}

if的部分很好理解，主要在于else部分。疑惑主要存在于，arg2并没有赋值，如何获得第二个参数的值。

add(3)//运行结果为function(arg2){    return arg1+arg2;}

//add(3)(4)等价于(仅帮助理解，但有区别）//如下取不到arg1的值，因为arg1的作用域在add()中)(function(arg2){    return arg1+arg2;})(4)

6. Write the ‘increase’ function that makes the following statements true (hint: please use JavaScript closure):

increase() === 0;increase() === 1;increase() === 2;…

var increase=(function(){    var count=0;    return function(){        return count++;    }})();

运用闭包，count变量不会被释放，在定义后会一直在内存中。到底定义的increase是什么样的。运行increase，可以看到，它实际是一个函数。

//increasefunction(){    return count++;}

7. Write a plugin that dynamically sets the background-image of a DIV element

called c-hero. This plugin should work generically for any website project. You may use CoffeeScript or regular JS syntax for your code.

You should consider best practices in your solution.

使用jQuery的plugin来写，另外，图片做示例不方便，下代码改为用颜色代替image。

<!DOCTYPE html><html><head lang="en">    <meta charset="UTF-8">    <title></title></head><body>    <div class="c-hero" style="height: 200px;width: 200px;">    </div></body><script src="http://apps.bdimg.com/libs/jquery/1.6.4/jquery.js"></script><script>    (function($){        $.fn.setBackground= function(options){            var settings = $.extend({                mobile: "#000",                desktop: "#f0f"            },options);            var img;            if(window.innerWidth < 768){                img = settings.mobile;            }else{                img = settings.desktop;            }            return this.css({                backgroundColor: img            });        }    })(jQuery);    $('.c-hero').setBackground();</script></html>

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