在对象上定义getter，以便所有未定义的属性查找返回“"” [英] Define getter on object so all undefined property lookups return ""

问题描述

基本上我需要能够这样做：

Basically I need to be able to do this:

var obj = {"foo":"bar"},
    arr = [];
with( obj ){
   arr.push( foo );
   arr.push( notDefinedOnObj ); // fails with 'ReferenceError: notDefinedOnObj is not defined'
}
console.log(arr); // ["bar", ""] <- this is what it should be.

我正在寻找 {} .__ defineGetter __ 或 {get} ，以便为所有未定义的属性getter返回一个空字符串（请注意，这与<$的属性不同） c $ c> undefined ）。

I'm looking for a "global" equivalent of {}.__defineGetter__ or {get} in order to return an empty string for all undefined property getters (note that this is different than a property that is undefined).

推荐答案

您可以创建 代理 每当未定义的属性返回空字符串访问。

You can create a Proxy to return an empty string whenever undefined properties are accessed.

app.js ：

var obj = {"foo":"bar"},
    arr = [],
    p = Proxy.create({
        get: function(proxy, name) {
            return obj[name] === undefined ? '' : obj[name];
        }
    });
arr.push( p.foo );
arr.push( p.notDefinedOnObj );

console.log(arr);

正如问题作者David Murdoch指出的那样，如果您使用的是节点v0.6.18（最新的稳定版本发布于写这篇文章的时间），你必须在运行脚本时传递 - harmony_proxies 选项：

As question author David Murdoch notes, if you are using node v0.6.18 (the latest stable release at the time this post was written), you must pass the --harmony_proxies option when you run the script:

$ node --harmony_proxies app.js
[ 'bar', '' ]

请注意，如果您将与一起使用，此解决方案将不，如：

Note that this solution will not work if you use with, as in:

var obj = {"foo":"bar"},
    arr = [],
    p = Proxy.create({
        get: function(proxy, name) {
            return obj[name] === undefined ? '' : obj[name];
        }
    });
with ( p ) {
   arr.push( foo ); // ReferenceError: foo is not defined
   arr.push( notDefinedOnObj );
}

console.log(arr);

带似乎没有调用代理的将代理添加到作用域链时，获取方法。

with does not seem to call the proxy's get method when adding the proxy to the scope chain.

注意：代理处理程序传递给 Proxy.create（）在这个示例中不完整。有关更多详细信息，请参阅代理：常见错误和误解。

Note: the proxy handler passed to Proxy.create() in this is example is incomplete. See Proxy: Common mistakes and misunderstanding for more details.

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