return statement in a function as class
I am confused about the return statement in a function that serves as a class. See the example code below:
<html>
<body>
<script type="text/javascript">
function test() {
this.abc = 'def';
return 3;
}
var mytest = new test();
document.write(mytest + ', ' + (typeof mytest) + ', ' + mytest.abc);
</script>
</bod开发者_JAVA百科y>
</html>
The code out put: [object Object], object, def.
Here is my question. I wrote 'return 3' in the test() function. Is this statement ignored when 'new test()' is called?
Thanks.
When you call a function with new, you're invoking it as a constructor which automatically returns the new object it constructs.
Your return 3; statement is ignored. What is returned is effectively:
{ abc:'def' }
...with an implicit reference to a prototype object, which in your example doesn't have any enumerable properties because you haven't given it any.
If you did:
mytest instanceof test;
...it would evaluate to true.
If you did:
function test() {
this.abc = 'def';
}
test.prototype.ghi = 'jkl';
var mytest = new test();
...you could then do:
mytest.ghi;
...which would give you the value 'jkl'.
When you use the new operator, you're using the function as a constructor, in that case for the return value:
- if it's not an object, it is ignored (like in your example)
- if it is an object, the object returned becomes the result of the whole
newexpression
So if you were to write
Test = function(arg) {
this.a = 1;
return arg;
}
var t1 = new Test(10);
var t2 = new Test({b: 2});
console.log(t1, t2)
// output:
// Test {a:1} Object {b: 2}
The new operator instantiates and returns an object. Here are some examples with its output:
(...)
var mytest = test();
document.write(mytest + ', ' + (typeof mytest) + ', ' + mytest.abc);
// 3, number, undefined
Or:
function test() {
this.abc = 'def';
this.getvalue = function(){
return 3;
}
}
var mytest = new test();
document.write(mytest.getvalue() + ', ' + (typeof mytest) + ', ' + mytest.abc);
// 3, object, def
you can do
function test(){
this.abc = "def"
this.ghi = function(){
return "jkl"
}
}or
function test(){
this.Class = function(){
this.def = "abc"
this.jkl = "ghi"
}
this.abc = "def"
this.ghi = function(){
return "jkl"
}
}
加载中,请稍侯......
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