blocks|key|3346893|text|没有办法在Javascript中设置这一点--为不存在的属性返回undefined是核心Javascript规范的一部分。参见the+discussion+for+this+similar+question。正如我在那里建议的，一种方法(尽管我不能真正推荐它)是定义一个全局getProperty函数：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3346894|function+getProperty(o,+prop)+{
++++if+(o[prop]+!==+undefined)+return+o[prop];
++++else+return+"my+default";
}

var+o+=+{
++++foo:+1
};

getProperty(o,+'foo');+//+1
getProperty(o,+'bar');+//+"my+default"|code-block|syntax|javascript|3346895|但这将导致大量的非标准代码，其他人很难阅读，并且在您期望或想要未定义的值的领域可能会产生意想不到的后果。最好是边走边检查：|3346896|var+someVar+=+o.someVar+%7C%7C+"my+default";|3346897|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/6511542/force-javascript-exception-error-when-reading-an-undefined-object-property^0|W|9|3T|B|1R|14|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|15|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

There isn't a way to set this in Javascript - returning <code>undefined</code> for non-existent properties is a part of the core Javascript spec. See <a href="https://stackoverflow.com/questions/6511542/force-javascript-exception-error-when-reading-an-undefined-object-property">the discussion for this similar question</a>. As I suggested there, one approach (though I can't really recommend it) would be to define a global <code>getProperty</code> function:

<pre><code>function getProperty(o, prop) {
 if (o[prop] !== undefined) return o[prop];
 else return "my default";
}

var o = {
 foo: 1
};

getProperty(o, 'foo'); // 1
getProperty(o, 'bar'); // "my default"
</code></pre>

But this would lead to a bunch of non-standard code that would be difficult for others to read, and it might have unintended consequences in areas where you'd expect or want an undefined value. Better to just check as you go: 

<pre><code>var someVar = o.someVar || "my default";
</code></pre>

blocks|key|1445376|text|我昨天看到一篇文章提到了一个Object.__noSuchMethod__属性：JavascriptTips我还没有机会去尝试它，所以我不知道浏览器的支持，但也许你可以以某种方式使用它？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1445377|entityMap|0|LINK|mutability|MUTABLE|url|http://code.google.com/p/jslibs/wiki/JavascriptTips#No_Such_Method_%2528_noSuchMethod_%2529^0|E|N|14|E|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@$9|Q|A|R|1|S]]|E|$]]|$1|F|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

I saw an article yesterday that mentions an <code>Object.__noSuchMethod__</code> property: <a href="http://code.google.com/p/jslibs/wiki/JavascriptTips#No_Such_Method_%28_noSuchMethod_%29" rel="nofollow">JavascriptTips</a> I've not had a chance to play around with it, so I don't know about browser support, but maybe you could use that in some way?

blocks|key|3061259|text|这听起来确实像是基于原型的对象的典型用法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3061260|//+define+a+new+type+of+object
var+foo+=+function()+{};++

//+define+a+default+attribute+and+value+that+all+objects+of+this+type+will+have
foo.prototype.attribute1+=+"defaultValue1";++

//+create+a+new+object+of+my+type
var+emptyObj+=+new+foo();
console.log(emptyObj.attribute1);+++++++//+outputs+defaultValue1|code-block|syntax|javascript|3061261|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This sure sounds like the typical use of protoype-based objects:

<pre><code>// define a new type of object
var foo = function() {}; 

// define a default attribute and value that all objects of this type will have
foo.prototype.attribute1 = "defaultValue1"; 

// create a new object of my type
var emptyObj = new foo();
console.log(emptyObj.attribute1); // outputs defaultValue1
</code></pre>

blocks|key|5131286|text|或者你可以试试这个|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5131287|dict+=+{
+'somekey':+'somevalue'
};

val+=+dict['anotherkey']+%7C%7C+'anotherval';|code-block|syntax|javascript|5131288|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Or you can try this

<pre><code>dict = {
 'somekey': 'somevalue'
};

val = dict['anotherkey'] || 'anotherval';
</code></pre>

blocks|key|3061270|text|我的代码是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3061271|function(s){
++++s+=+{
++++++++top:+s.top+%7C%7C+100,++++//+default+value+or+s.top
++++++++left:+s.left+%7C%7C+300,++//+default+value+or+s.left
++++}
++++alert(s.top)
}|code-block|syntax|javascript|3061272|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

my code is:

<pre><code>function(s){
 s = {
 top: s.top || 100, // default value or s.top
 left: s.left || 300, // default value or s.left
 }
 alert(s.top)
}
</code></pre>

blocks|key|1445480|text|这实际上可以用Object.create来实现。它不适用于“未定义”的属性。而是对于已被赋予默认值的那些。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1445481|var+defaults+=+{
++++a:+'test1',
++++b:+'test2'
};|code-block|syntax|javascript|1445482|然后，当您创建属性对象时，可以使用Object.create完成此操作|1445483|properties+=+Object.create(defaults);|1445484|现在您将拥有两个对象，其中第一个对象为空，但原型指向defaults对象。要测试：|1445485|console.log('Unchanged',+properties);
properties.a+=+'updated';
console.log('Updated',+properties);
console.log('Defaults',+Object.getPrototypeOf(properties));|1445486|entityMap^0|7|D|0|0|H|D|0|0|Q|8|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|12|8|@$9|13|A|14|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|T|$]]

This is actually possible to do with <code>Object.create</code>. It will not work for "non defined" properties. But for the ones that has been given a default value.

<pre><code>var defaults = {
 a: 'test1',
 b: 'test2'
};
</code></pre>

Then when you create your properties object you do it with <code>Object.create</code>

<pre><code>properties = Object.create(defaults);
</code></pre>

Now you will have two object where the first object is empty, but the prototype points to the <code>defaults</code> object. To test:

<pre><code>console.log('Unchanged', properties);
properties.a = 'updated';
console.log('Updated', properties);
console.log('Defaults', Object.getPrototypeOf(properties));
</code></pre>

blocks|key|3061320|text|我很惊讶还没有人提到三元运算符。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3061321|var+emptyObj+=+{a:'123',+b:'234',+c:0};
var+defaultValue+=+'defaultValue';
var+attr+=+'someNonExistAttribute';
emptyObj.hasOwnProperty(attr)+?+emptyObj[attr]+:+defaultValue;//=>+'defaultValue'


attr+=+'c';+//+=>+'c'
emptyObj.hasOwnProperty(attr)+?+emptyObj[attr]+:+defaultValue;+//+=>+0|code-block|syntax|javascript|3061322|这样，即使'c‘的值是0，它仍然会得到正确的值。|3061323|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I'm surprised nobody has mentioned ternary operator yet.

<pre><code>var emptyObj = {a:'123', b:'234', c:0};
var defaultValue = 'defaultValue';
var attr = 'someNonExistAttribute';
emptyObj.hasOwnProperty(attr) ? emptyObj[attr] : defaultValue;//=&gt; 'defaultValue'


attr = 'c'; // =&gt; 'c'
emptyObj.hasOwnProperty(attr) ? emptyObj[attr] : defaultValue; // =&gt; 0
</code></pre>

In this way, even if the value of 'c' is 0, it will still get the correct value.

blocks|key|1445561|text|自从我几年前提出这个问题以来，事情进展得很顺利。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1445562|代理是ES6的一部分。以下示例适用于Chrome,+Firefox,+Safari+and+Edge|offset|length|1445563|let+handler+=+{
++get:+function(target,+name)+{
++++return+target.hasOwnProperty(name)+?+target[name]+:+42;
++}
};

let+emptyObj+=+{};
let+p+=+new+Proxy(emptyObj,+handler);

p.answerToTheUltimateQuestionOfLife;+//=>+42|code-block|syntax|javascript|1445564|1445565|在Mozilla's+documentation+on+Proxies上阅读更多内容。|1445566|entityMap|0|LINK|mutability|MUTABLE|url|https://kangax.github.io/compat-table/es6/#test-Proxy|1|https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Proxy^0|0|I|W|0|0|0|0|1|Y|1|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Y|8|@]|9|@$D|Z|E|10|1|11]]|A|$]]|$1|F|3|G|5|H|7|12|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|13|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|14|8|@]|9|@$D|15|E|16|1|17]]|A|$]]|$1|N|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|O|$P|$5|Q|R|S|A|$T|U]]|V|$5|Q|R|S|A|$T|W]]]]

Since I asked the question several years ago things have progressed nicely.
Proxies are part of ES6. The following example works in <a href="https://kangax.github.io/compat-table/es6/#test-Proxy" rel="noreferrer">Chrome, Firefox, Safari and Edge</a>:
<pre><code>let handler = {
 get: function(target, name) {
 return target.hasOwnProperty(name) ? target[name] : 42;
 }
};

let emptyObj = {};
let p = new Proxy(emptyObj, handler);

p.answerToTheUltimateQuestionOfLife; //=&gt; 42
</code></pre>
<hr />
Read more in <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Proxy" rel="noreferrer">Mozilla's documentation on Proxies</a>.

blocks|key|1445599|text|一种方法是获取一个默认对象并将其与目标对象合并。目标对象将覆盖默认对象中的值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1445600|jQuery具有完成此任务的.extend()方法。但是，不需要jQuery，因为有一些普通的JS实现，可以在这里找到：|offset|length|style|CODE|1445601|http://gomakethings.com/vanilla-javascript-version-of-jquery-extend/|1445602|entityMap|0|LINK|mutability|MUTABLE|url^0|0|E|9|0|0|1W|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|U|8|@]|9|@$D|V|E|W|1|X]]|A|$]]|$1|J|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|I]]]]

One approach would be to take a defaults object and merge it with the target object. The target object would override values in the defaults object.

jQuery has the <code>.extend()</code> method that does this. jQuery is not needed however as there are vanilla JS implementations such as can be found here:

<a href="http://gomakethings.com/vanilla-javascript-version-of-jquery-extend/" rel="nofollow">http://gomakethings.com/vanilla-javascript-version-of-jquery-extend/</a>

blocks|key|1445628|text|我认为最简单的方法是使用Object.assign。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1445629|如果你有这个类：|1445630|class+MyHelper+{
++++constructor(options)+{
++++++++this.options+=+Object.assign({
++++++++++++name:+"John",
++++++++++++surname:+"Doe",
++++++++++++birthDate:+"1980-08-08"
++++++++},+options);
++++}
}|code-block|syntax|javascript|1445631|你可以这样使用它：|1445632|let+helper+=+new+MyHelper({+name:+"Mark"+});
console.log(helper.options.surname);+//+this+will+output+"Doe"|1445633|文档(使用polyfill)：https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Global_Objects/Object/assign|1445634|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Global_Objects/Object/assign^0|C|D|0|0|0|0|0|F|2J|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|17|8|@]|D|@$9|18|A|19|1|1A]]|E|$]]|$1|S|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

I think the simplest approach is using <code>Object.assign</code>.

If you have this Class:

<pre><code>class MyHelper {
 constructor(options) {
 this.options = Object.assign({
 name: "John",
 surname: "Doe",
 birthDate: "1980-08-08"
 }, options);
 }
}
</code></pre>

You can use it like this:

<pre><code>let helper = new MyHelper({ name: "Mark" });
console.log(helper.options.surname); // this will output "Doe"
</code></pre>

Documentation (with polyfill):
<a href="https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Global_Objects/Object/assign" rel="noreferrer">https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Global_Objects/Object/assign</a>

blocks|key|5131569|text|使用解构(ES6中的新增功能)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5131570|有一个很棒的documentation+by+Mozila，还有一个很棒的blog+post，它能比我更好地解释语法。|offset|length|5131571|回答你的问题|5131572|var+emptyObj+=+{};
const+{+nonExistingAttribute+=+defaultValue+}+=+emptyObj;
console.log(nonExistingAttribute);+//+defaultValue|code-block|syntax|javascript|5131573|更进一步|5131574|我能重命名这个变量吗?I+this+variable？好的!|5131575|const+{+nonExistingAttribute:+coolerName+=+15}+=+emptyObj;
console.log(coolerName);+//+15|5131576|嵌套数据呢？来吧！|5131577|var+nestedData+=+{
++++name:+'Awesome+Programmer',
++++languages:+[
++++++++{
++++++++++++name:+'javascript',
++++++++++++proficiency:+4,
++++++++}
++++],
++++country:+'Canada',
};

var+{name:+realName,+languages:+[{name:+languageName}]}+=+nestedData+;

console.log(realName);+//+Awesome+Programmer
console.log(languageName);+//+javascript|5131578|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment|1|http://wesbos.com/destructuring-default-values/^0|0|6|N|0|11|9|1|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@]|9|@$D|18|E|19|1|1A]|$D|1B|E|1C|1|1D]]|A|$]]|$1|F|3|G|5|6|7|1E|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|1F|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|1G|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1H|8|@]|9|@]|A|$]]|$1|Q|3|R|5|J|7|1I|8|@]|9|@]|A|$K|L]]|$1|S|3|T|5|6|7|1J|8|@]|9|@]|A|$]]|$1|U|3|V|5|J|7|1K|8|@]|9|@]|A|$K|L]]|$1|W|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|13]]|14|$5|Z|10|11|A|$12|15]]]]

Use destructuring (new in ES6)

There is great <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment" rel="noreferrer">documentation by Mozila</a> as well as a fantastic <a href="http://wesbos.com/destructuring-default-values/" rel="noreferrer">blog post</a> that explains the syntax better than I can.

<h3>To Answer Your Question</h3>

<pre><code>var emptyObj = {};
const { nonExistingAttribute = defaultValue } = emptyObj;
console.log(nonExistingAttribute); // defaultValue
</code></pre>

<h3>Going Further</h3>

Can I rename this variable? Sure!

<pre><code>const { nonExistingAttribute: coolerName = 15} = emptyObj;
console.log(coolerName); // 15
</code></pre>

What about nested data? Bring it on!

<pre><code>var nestedData = {
 name: 'Awesome Programmer',
 languages: [
 {
 name: 'javascript',
 proficiency: 4,
 }
 ],
 country: 'Canada',
};

var {name: realName, languages: [{name: languageName}]} = nestedData ;

console.log(realName); // Awesome Programmer
console.log(languageName); // javascript
</code></pre>

blocks|key|3347226|text|Object.withDefault+=+(defaultValue,o={})+=>+{
++return+new+Proxy(o,+{
++++get:+(o,+k)+=>+(k+in+o)+?+o[k]+:+defaultValue+
++});
}

o+=+Object.withDefault(42);
o.x++//=>+42

o.x+=+10
o.x++//=>+10
o.xx+//=>+42|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3347227|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>Object.withDefault = (defaultValue,o={}) =&gt; {
 return new Proxy(o, {
 get: (o, k) =&gt; (k in o) ? o[k] : defaultValue 
 });
}

o = Object.withDefault(42);
o.x //=&gt; 42

o.x = 10
o.x //=&gt; 10
o.xx //=&gt; 42
</code></pre>

blocks|key|3347314|text|最简单的解决方案：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3347315|dict+=+{'first':+1,
++++++++'second':+2,
++++++++'third':+3}|code-block|syntax|javascript|3347316|现在,|3347317|dict['last']+%7C%7C+'Excluded'|3347318|将返回默认值'Excluded‘。|3347319|entityMap^0|0|9|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|Y|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|R|$]]

Simplest of all Solutions:

<pre><code>dict = {'first': 1,
 'second': 2,
 'third': 3}
</code></pre>

Now,

<pre><code>dict['last'] || 'Excluded'
</code></pre>

will return 'Excluded', which is the default value.

blocks|key|3347368|text|实现这一点的方法是使用object.assign函数|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3347369|const+defaultProperties+=+{+'foo':+'bar',+'bar':+'foo'+};
const+overwriteProperties+=+{+'foo':+'foo'+};
const+newObj+=+Object.assign({},+defaultProperties,+overwriteProperties);
console.log(defaultProperties);++//+{"foo":+"bar",+"bar":+"foo"}
console.log(overwriteProperties);++//+{+"foo":+"foo"+};
console.log(newObj);++//+{+"foo":+"foo",+"bar":+"foo"+}|code-block|syntax|javascript|3347370|entityMap^0|B|D|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

The way I achieve this is with the <code>object.assign</code> function

<pre><code>const defaultProperties = { 'foo': 'bar', 'bar': 'foo' };
const overwriteProperties = { 'foo': 'foo' };
const newObj = Object.assign({}, defaultProperties, overwriteProperties);
console.log(defaultProperties); // {"foo": "bar", "bar": "foo"}
console.log(overwriteProperties); // { "foo": "foo" };
console.log(newObj); // { "foo": "foo", "bar": "foo" }
</code></pre>

blocks|key|3061756|text|我来这里寻找解决方案，因为报头匹配我的问题描述，但这不是我想要的，但我得到了我的问题的解决方案(我想要一个属性的默认值，它将是动态的，比如日期)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3061757|let+Blog+=+{
title++:+String,
image++:+String,
body+++:+String,
created:+{type:+Date,+default:+Date.now}
}|code-block|syntax|javascript|3061758|上面的代码是我最终选择的解决方案。|3061759|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I came here looking for a solution because the header matched my problem description but it isn't what i was looking for but i got a solution to my problem(I wanted to have a default value for an attribute which would be dynamic something like date).

<pre><code>let Blog = {
title : String,
image : String,
body : String,
created: {type: Date, default: Date.now}
}
</code></pre>

The above code was the solution for which i finally settled.

blocks|key|1445962|text|在我看来，这似乎是最简单、最易读的方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1445963|let+options+=+{name:"James"}
const+default_options+=+{name:"John",+surname:"Doe"}

options+=+Object.assign({},+default_options,+options)|code-block|syntax|javascript|1445964|Object.assign()+reference|offset|length|1445965|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign^0|0|0|0|P|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

This seems to me the most simple and readable way of doing so:

<pre><code>let options = {name:"James"}
const default_options = {name:"John", surname:"Doe"}

options = Object.assign({}, default_options, options)
</code></pre>

<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign" rel="noreferrer">Object.assign() reference</a>

blocks|key|1446006|text|var+obj+=+{
++a:+2,
++b:+4
};

console.log(obj);

-->+{a:+2,+b:+4}

function+applyDefaults(obj)+{
++obj.a+%7C%7C=+10;
++obj.b+%7C%7C=+10;
++obj.c+%7C%7C=+10;
}

//+do+some+magic
applyDefaults(obj);

console.log(obj);

-->+{a:+2,+b:+4,+c:+10}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1446007|这之所以有效，是因为|unstyled|1446008|undefined+%7C%7C+"1111111"+-->+"1111111"
"0000000"+%7C%7C+"1111111"+-->+"0000000"|1446009|as+null、undefined、NaN、0、""+(空字符串)、false本身都被认为等同于false+(falsy)。任何其他内容都是true+(truthy)。|offset|length|style|CODE|1446010|请注意，这不是跨浏览器和nodejs版本的统一支持(请自行确认)。|1446011|因此，两种麻烦的情况是空字符串""和0+(零)。如果不覆盖它们很重要，您可能需要将其重写为：|1446012|if+(typeof+obj.d+==+"undefined")+obj.d+=+"default"|1446013|这也将在浏览器之间得到更好的支持。|1446014|或者，您可以将其编写为：|1446015|obj.d+??=+"default"|1446016|这是仅适用于null或undefined+(nullish)的值的nullish+assignment，这些值不是空字符串的一部分。然而，这再次减少了对跨浏览器的支持。|1446017|另请参阅Mozilla官方网站-+Assigning+a+default+value+to+a+variable。|1446018|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Glossary/Falsy|1|https://developer.mozilla.org/en-US/docs/Glossary/Truthy|2|https://developer.mozilla.org/en-US/docs/Glossary/Nullish|3|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment|4|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator#assigning_a_default_value_to_a_variable^0|0|0|0|3|4|8|9|I|3|M|1|O|2|Y|5|1Y|4|1J|5|0|24|6|1|0|0|F|2|I|1|0|0|0|0|0|6|4|B|9|M|7|2|X|I|3|0|H|13|4|0^^$0|@$1|2|3|4|5|6|7|1K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|1L|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|1M|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|1N|8|@$K|1O|L|1P|M|N]|$K|1Q|L|1R|M|N]|$K|1S|L|1T|M|N]|$K|1U|L|1V|M|N]|$K|1W|L|1X|M|N]|$K|1Y|L|1Z|M|N]|$K|20|L|21|M|N]]|9|@$K|22|L|23|1|24]|$K|25|L|26|1|27]]|A|$]]|$1|O|3|P|5|F|7|28|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|29|8|@$K|2A|L|2B|M|N]|$K|2C|L|2D|M|N]]|9|@]|A|$]]|$1|S|3|T|5|6|7|2E|8|@]|9|@]|A|$B|C]]|$1|U|3|V|5|F|7|2F|8|@]|9|@]|A|$]]|$1|W|3|X|5|F|7|2G|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|2H|8|@]|9|@]|A|$B|C]]|$1|10|3|11|5|F|7|2I|8|@$K|2J|L|2K|M|N]|$K|2L|L|2M|M|N]]|9|@$K|2N|L|2O|1|2P]|$K|2Q|L|2R|1|2S]]|A|$]]|$1|12|3|13|5|F|7|2T|8|@]|9|@$K|2U|L|2V|1|2W]]|A|$]]|$1|14|3|-4|5|F|7|2X|8|@]|9|@]|A|$]]]|15|$16|$5|17|18|19|A|$1A|1B]]|1C|$5|17|18|19|A|$1A|1D]]|1E|$5|17|18|19|A|$1A|1F]]|1G|$5|17|18|19|A|$1A|1H]]|1I|$5|17|18|19|A|$1A|1J]]]]

<pre><code>var obj = {
 a: 2,
 b: 4
};

console.log(obj);

--&gt; {a: 2, b: 4}

function applyDefaults(obj) {
 obj.a ||= 10;
 obj.b ||= 10;
 obj.c ||= 10;
}

// do some magic
applyDefaults(obj);

console.log(obj);

--&gt; {a: 2, b: 4, c: 10}
</code></pre>
This works because
<pre><code>undefined || &quot;1111111&quot; --&gt; &quot;1111111&quot;
&quot;0000000&quot; || &quot;1111111&quot; --&gt; &quot;0000000&quot;
</code></pre>
as <code>null</code>, <code>undefined</code>, <code>NaN</code>, <code>0</code>, <code>&quot;&quot;</code> (Empty String), <code>false</code> itself, are all considered to be equivalent to false (<a href="https://developer.mozilla.org/en-US/docs/Glossary/Falsy" rel="nofollow noreferrer">falsy</a>). Anything else is <code>true</code> (<a href="https://developer.mozilla.org/en-US/docs/Glossary/Truthy" rel="nofollow noreferrer">truthy</a>).
Note that this is not uniformly supported across browsers and nodejs versions (confirm for yourself).
So two troublesome cases are the empty String <code>&quot;&quot;</code> and <code>0</code> (zero). If it is important not to override those, you might need to rewrite this as:
<pre><code>if (typeof obj.d == &quot;undefined&quot;) obj.d = &quot;default&quot;
</code></pre>
This will be better supported across browsers also.
Alternatively you could write this as:
<pre><code>obj.d ??= &quot;default&quot;
</code></pre>
This is the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment" rel="nofollow noreferrer">nullish assignment</a> which applies only to values that are <code>null</code> or <code>undefined</code> (<a href="https://developer.mozilla.org/en-US/docs/Glossary/Nullish" rel="nofollow noreferrer">nullish</a>) - of which the empty string is not part. However, this has again a diminished cross-browser support.
See also on the official Mozilla Website - <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator#assigning_a_default_value_to_a_variable" rel="nofollow noreferrer">Assigning a default value to a variable</a>.

Is there a way to set the default attribute of a Javascript object such that:
<pre><code>let emptyObj = {};
// do some magic
emptyObj.nonExistingAttribute // =&gt; defaultValue
</code></pre>

Set default value of Javascript object attributes

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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有没有一种方法可以设置Javascript对象的默认属性，以便：let emptyObj = {};// do some magicemptyObj.nonExistingAttribute // => defaultValue

问设置Javascript对象属性的默认值
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