Javascript引用与价值 [英] Javascript by reference vs. by value

问题描述

我正在寻找一些很好的综合阅读材料，当Javascript按值传递某些内容时，以及何时通过引用和修改传递的项目时会影响函数外部的值，何时不会。我也有兴趣在通过引用和值分配给另一个变量时是否遵循任何不同的规则而不是作为函数参数传递。

I'm looking for some good comprehensive reading material on when Javascript passes something by value and when by reference and when modifying a passed item affects the value outside a function and when not. I'm also interested in when assigning to another variable is by reference vs. by value and whether that follows any different rules than passing as a function parameter.

我是做了很多搜索，找到了很多具体的例子（其中很多都是在SO上），我可以从中开始拼凑出真正规则的部分，但我还没有找到一个描述一切的单一的，写得很好的文档。

I've done a lot of searching and find lots of specific examples (many of them here on SO) from which I can start to piece together pieces of the real rules, but I haven't yet found a single, well written document that describes it all.

此外，语言中是否有方法可以控制是通过引用还是通过值传递某些内容？

Also, are there ways in the language to control whether something is passed by reference or by value?

以下是我想要了解的一些问题类型。这些只是示例 - 我实际上是在了解语言所遵循的规则，而不仅仅是具体示例的答案。但是，这里有一些例子：

Here are some of the types of questions I want to understand. These are just examples - I'm actually looking to understand the rules the language goes by, not just the answers to specific examples. But, here are some examples:

function f(a,b,c) {
   a = 3;
   b.push("foo");
   c.first = false;
}

var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
f(x,y,z);

对于所有不同类型，x，y和z的内容何时更改为f的范围？

When are the contents of x, y and z changed outside the scope of f for all the different types?

function f() {
    var a = ["1", "2", "3"];
    var b = a[1];
    a[1] = "4";
    // what is the value of b now for all possible data types that the array in "a" might hold?
}

function f() {
    var a = [{yellow: "blue"}, {red: "cyan"}, {green: "magenta"}];
    var b = a[1];
    a[1].red = "tan";
    // what is the value of b now and why?
    b.red = "black";
    // did the value of a[1].red change when I assigned to b.red?
}

如果我想制作一个完全独立的对象副本（不提供任何参考） ），最好的做法是什么？

If I want to make a fully independent copy of an object (no references whatsoever), what's the best practice way to do that?

推荐答案

我的理解是这实际上非常简单：

My understanding is that this is actually very simple:

• Javascript 始终按值传递，但当变量引用对象（包括数组）时，值为对象的引用。


• 更改变量 never 的值会更改基础图元或对象，它只是将变量指向新的图元或对象。


• 但是，更改变量引用的对象的属性确实会更改基础对象。

• Javascript is always pass by value, but when a variable refers to an object (including arrays), the "value" is a reference to the object.
• Changing the value of a variable never changes the underlying primitive or object, it just points the variable to a new primitive or object.
• However, changing a property of an object referenced by a variable does change the underlying object.

所以，要完成你的一些例子：

So, to work through some of your examples:

function f(a,b,c) {
    // Argument a is re-assigned to a new value.
    // The object or primitive referenced by the original a is unchanged.
    a = 3;
    // Calling b.push changes its properties - it adds
    // a new property b[b.length] with the value "foo".
    // So the object referenced by b has been changed.
    b.push("foo");
    // The "first" property of argument c has been changed.
    // So the object referenced by c has been changed (unless c is a primitive)
    c.first = false;
}

var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
f(x,y,z);
console.log(x, y, z.first); // 4, ["eeny", "miny", "mo", "foo"], false

示例2：

var a = ["1", "2", {foo:"bar"}];
var b = a[1]; // b is now "2";
var c = a[2]; // c now references {foo:"bar"}
a[1] = "4";   // a is now ["1", "4", {foo:"bar"}]; b still has the value
              // it had at the time of assignment
a[2] = "5";   // a is now ["1", "4", "5"]; c still has the value
              // it had at the time of assignment, i.e. a reference to
              // the object {foo:"bar"}
console.log(b, c.foo); // "2" "bar"

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