javascript - 如何在函数内部将未定义的 var 转换为对象

Question

有些事情我找不到答案或解释。让我们以下面的代码为例:

function fn(x){
  x = {value: 10};
}
var a;
fn(a);

alert(a.value); //a is undefined

当我们通过该函数传递它时，a = {value: 10}; 不应该吗？

Answer 1

x 是本地范围的。您只传递值 而不是引用。因此，您可能需要像这样返回并分配:

function fn(x){
  x = {value: 10};
  return x;
}
var a;
a = fn(a);

来自awesome article :

When passing in a primitive type variable like a string or a number, the value is passed in by value. This means that any changes to that variable while in the function are completely separate from anything that happens outside the function.

function myfunction(x)
{
    // x is equal to 4
    x = 5;
    // x is now equal to 5
}

var x = 4;
alert(x); // x is equal to 4
myfunction(x); 
alert(x); // x is still equal to 4

Passing in an object, however, passes it in by reference. In this case, any property of that object is accessible within the function.

function myobject()
{
    this.value = 5;
}
var o = new myobject();
alert(o.value); // o.value = 5
function objectchanger(fnc)
{
    fnc.value = 6;
}
objectchanger(o);
alert(o.value); // o.value is now equal to 6