Copying, passing, and comparing by value 的 疑惑

[moliu]

var n = 1;
var m = n;
function add_to_total(total, x)

{

total = total + x;
document.write(total+"</br>");
document.write(x+"</br>")
}



add_to_total(n, m);//输出 2 和 1，
document.write(n+"</br>")//输出 1
document.write(m+"</br>")//输出 1
// 后面讲解中，提到 n=1 m=2 ,可上面的程序证明，却不是这样。请达人讲解，谢谢

附：js权威指南中的讲解
var n = 1; // Variable n holds the value 1

var m = n; // Copy by value: variable m holds a distinct value 1



// Here's a function we'll use to illustrate passing by value

// As we'll see, the function doesn't work the way we'd like it to

function add_to_total(total, x)

{

total = total + x; // This line changes only the internal copy of total

}



// Now call the function, passing the numbers contained in n and m by value.

// The value of n is copied, and that copied value is named total within the

// function. The function adds a copy of m to that copy of n. But adding

// something to a copy of n doesn't affect the original value of n outside

// of the function. So calling this function doesn't accomplish anything.

add_to_total(n, m);



// Now, we'll look at comparison by value.

// In the following line of code, the literal 1 is clearly a distinct numeric

// value encoded in the program. We compare it to the value held in variable

// n. In comparison by value, the bytes of the two numbers are checked to

// see if they are the same.

if (n == 1) m = 2; // n contains the same value as the literal 1; m is now 2

[friendly_]

if (n == 1) m = 2; // n contains the same value as the literal 1; m is now 2

[moliu]

var n = 1;
var m = n;
function add_to_total(total, x)

{

total = total + x;
document.write(total+" </br>");
document.write(x+" </br>")
}
add_to_total(n, m);//输出 2 和 1
//这如何解释？

[moliu]

[Quote=引用 1 楼 fandelei1982 的回复:]
if (n == 1) m = 2;  // n contains the same value as the literal 1; m is now 2

[/Quote]
还是不明白。为什么 m=2 ?

[moliu]

if (n == 1) m = 2; // n contains the same value as the literal 1; m is now 2
//n 值确实 为 1 ,给 m 重新赋值 2，m is now 2 。原来 m 值 为 1 。
//原来是这样理解的，以为 m 的值为 2 ，是函数执行后的结果。这样的理解是错误的。