代码改变世界

Javascript 关于array、object、+= 基本理解

2016-08-09 13:48  yojiaku  阅读(238)  评论(0)  编辑  收藏  举报 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8"/>
    <title>Example</title>
</head>
<body>


    <!-- the best method to quote javascript is putting it before the tag </body> -->
    <script type="text/javascript">
        /* declare an array*/
        var beatles = Array("John","Paul","George","Ringo");
        /* we also can declare an array like this:
            var beatles = ["John","Paul","George","Ringo"];

            the most important thing is : you can use string to instead label subscript==>
            var lennon = Array();
            lennon["name"] = "John";
            lennon["year"] = 1940;
            lennon["living"] = false;
            However, in fact, you just add three attributes to array lennon. So, do not use this method.

            In fact, Array is a native object:
            var array = []; <=> var array = new Array();
            So, if you want to know how long the array is, you can use the function which includes in the Array(): array.length;
        */

        /* build an object */
        var lennon = object();
        lennon.name = "John"; lennon.year = 1940; lennon.living = false;
        /* It is noteworthy that Object via "." to obtain attributes.
            And there is a much easier way to build an Objet:
            var lennon = {name:"Jhon", year:1940, living:false};

            We can also put object into array, and via this way to obtain it:
            var beatles = Array();
            beatles[0] = lennon;
            Then we can use "beatles[0].name" to obtain the value "Jhon".
            It is really easier to remember.

            We can also put one object into another object:
            var beatles = {};
            beatles.vocalist = lennon;
            Then we can use "beatles.vocalist.name" to obtain the value "John", use "beatles.vocalist.year" to obtain the value "1940" and so on.
        */

        /* A very useful operator : += */
        var year = 2010;
        var message = "This year is ";
        message += year;
        alert(message);
        /* It will ouput : This year is 2010 ;
            The operator "+=" can mean "addition & assignment" , and it can also mean "concatention & assignment".
        */

        /* global variable and local variable (you can use the keyword "var" to set scope for variables)*/
        function square(num)
        {
            total = num * num;
            return total;
        }
        var total = 50;
        var number = square(20);
        alert(total);
        /* It will output : 400 ; it means the total is a global variable and the function square() change the value of total and this is not my intention.
        However, we can use "var" to let the total which in square() become a local variable:
        function square(num)
        {
            var total = num * num;
            return total;
        }
        Then we invoke squre():
        var total = 50;
        var number = squre(20);
        alert(total);
        And this will output : 50. We can see it clearly that we protect the global variable "total".
        */

        /* native object */
        var number2 = 7.561;
        var number2 = Math.round(number2);
        alert(number2);
        /* It will output: 8. var number2 is an instance of native object "Math", then you can use function round() which is included in "Math" */
        var current_date = new Date();
        var today = current_date.getDay();
        /* The same to "Math", Date is also a native object, it provides getDay(), getHours(), getMonth(). */

    </script>
</body>
</html>

所有的解释都清楚地写在代码中了，o(*￣▽￣*)o！