blocks|key|4236618|text|你可以手动做，我不明白为什么“没有循环”。jQuery也以这样或那样的方式循环：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4236619|var+result+=+{};
for+(var+key+in+theSource)+{
++if+(theTarget[key])+result[key]+=+theSource[key];
}|code-block|syntax|javascript|4236620|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|$]]

You can do it manually, I don't understand why "no loops". jQuery is looping too in some way or another:

<pre><code>var result = {};
for (var key in theSource) {
 if (theTarget[key]) result[key] = theSource[key];
}
</code></pre>

blocks|key|736642|text|下面是一行代码:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|736643|for(var+propertyName+in+theTarget)theTarget[propertyName]&&(theTarget[propertyName]=theSource[propertyName]);|code-block|syntax|javascript|736644|使用underscore.js，您可以执行以下操作：|offset|length|style|CODE|736645|_(theTarget).extend(_(theSource).pick(+_(theTarget).keys()+));|736646|entityMap^0|0|0|2|D|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

here's the one-liner :)

<pre><code>for(var propertyName in theTarget)theTarget[propertyName]&amp;&amp;(theTarget[propertyName]=theSource[propertyName]);
</code></pre>

with <code>underscore.js</code> you can do:

<pre><code>_(theTarget).extend(_(theSource).pick( _(theTarget).keys() ));
</code></pre>

blocks|key|1878734|text|你必须检查source的键，检查它们是否存在(不是它们是否真实，因为这不会复制0，''，false，null，undefined，NaN)，并将那个值复制到结果对象中。因为你不想覆盖源或目标，所以不要修改它们。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1878735|jQuery.overwriteOnlyExisting+=+function+(source,+target)+{
++++var+result+=+{},+key;
++++for+(key+in+target)+{
++++++++result[key]+=+key+in+source+?+source[key]+:+target[key];
++++}
++++return+result
};|code-block|syntax|javascript|1878736|entityMap^0|5|6|13|1|15|2|18|5|1E|4|1J|9|1T|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|L|$]]

You have to go through the <code>source</code>'s keys, check if they exists (not if they are truthy as that wont copy over <code>0</code>, <code>''</code>, <code>false</code>, <code>null</code>, <code>undefined</code>, <code>NaN</code>) and copy that value to the result object. Since you don't want to overwrite the source or target, don't modify them.

<pre><code>jQuery.overwriteOnlyExisting = function (source, target) {
 var result = {}, key;
 for (key in target) {
 result[key] = key in source ? source[key] : target[key];
 }
 return result
};
</code></pre>

blocks|key|4236707|text|好的!眼线笔！没有可见的循环！|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4236708|Object.keys(theTarget).map(function(a){+if(theSource[a])+theTarget[a]=theSource[a]})|code-block|syntax|javascript|4236709|虽然map的源代码中有一个循环，但我敢肯定。但我认为这是在没有可见的循环构造的情况下实现它的唯一方法。尽管它滥用了javascript的全局命名空间，因此是肮脏的。|4236710|好吧，更好的是：|4236711|Object.keys(theTarget).map(function(a){+if(Object.keys(theSource).indexOf(a))+theTarget[a]=theSource[a]})|4236712|更简洁|4236713|keys(theTarget).map(function(a){+if(a+in+theSource)+theTarget[a]=theSource[a]})+|4236714|尽管keys()和Array#indexOf在旧的ecma版本中不起作用。|4236715|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|Z|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|10|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

OK! oneliner! no visible loops!

<pre><code>Object.keys(theTarget).map(function(a){ if(theSource[a]) theTarget[a]=theSource[a]})
</code></pre>

Though map has a loop in it's source I'm sure. but I think this the only way to do it without a visible loop construct. though it abuses javascript's global namespace and is therefore dirty.

Okay, even better:

<pre><code>Object.keys(theTarget).map(function(a){ if(Object.keys(theSource).indexOf(a)) theTarget[a]=theSource[a]})
</code></pre>

and more concise

<pre><code>keys(theTarget).map(function(a){ if(a in theSource) theTarget[a]=theSource[a]}) 
</code></pre>

though keys() and Array#indexOf wont work in older ecma versions.

blocks|key|1878747|text|var+theSource+=+{
+field1:+"TEXT",
+field2:+"VAL",
+field3:+"ZZ",
+field4:+"4",
+field5:+"5"
},
theTarget+=+{
+field2:+"0",
+field3:+"",
+field4:+null,
+field5:+undefined
};

var+overrideExistingProperties+=+function+(theTarget,+theSource){
++++for+(var+property+in+theSource)
++++++++if+(theSource.hasOwnProperty(property)+&&+theTarget.hasOwnProperty(property))
++++++++++++theTarget[property]+=+theSource[property];
};

overrideExistingProperties(theTarget,+theSource);
result+=+theTarget;+//if+you+don't+want+to+clone|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1878748|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>var theSource = {
 field1: "TEXT",
 field2: "VAL",
 field3: "ZZ",
 field4: "4",
 field5: "5"
},
theTarget = {
 field2: "0",
 field3: "",
 field4: null,
 field5: undefined
};

var overrideExistingProperties = function (theTarget, theSource){
 for (var property in theSource)
 if (theSource.hasOwnProperty(property) &amp;&amp; theTarget.hasOwnProperty(property))
 theTarget[property] = theSource[property];
};

overrideExistingProperties(theTarget, theSource);
result = theTarget; //if you don't want to clone
</code></pre>

Is there a way in javascript 
to assign values of named fields in an object to the same fields of another object, if and only if the target field exists. I.e. overwrite old values, NOT adding new values, using ideomatic constructs, one-liners (special for javascript and/or jQuery) and in no way loops, even for-in.

<pre><code>var theSource = {
 field1: "TEXT",
 field2: "VAL",
 field3: "ZZ",
 field4: "4",
 field5: "5"
},
theTarget = {
 field2: "0",
 field3: "",
 field4: null,
 field5: undefined
};
</code></pre>

Something like

<pre><code>var result = jQuery.overwriteOnlyExisting(theTarget, theSource);
result === {
 field2: "VAL"
 field3: "ZZ"
... 
}
</code></pre>

NO field1 AND old fields after field3 are preserved.

<code>jQuery.extend</code> - can overwrite values, but it copies new fields as well.

Which options do we have here?

<a href="http://jsbin.com/owivat/1/edit">http://jsbin.com/owivat/1/edit</a> (underscore) - I like this, now the time to find the jquery way.

Results:

<pre><code>_.extend(theTarget, _(theSource).pick(_(theTarget).keys()));
</code></pre>

142,850 Ops/sec

<pre><code>Object.keys(theTarget).map(function(a) { if (a in theSource) theTarget[a] = theSource[a]; });
</code></pre>

403,243 Ops/sec

Javascript: Overwrite ONLY existing fields in one object with the fields in another

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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当且仅当目标字段存在时，javascript中有没有一种方法可以将一个对象中的命名字段的值分配给另一个对象的相同字段。即覆盖旧值，不添加新值，使用表意结构，单行程序(专门用于javascript和/或jQuery)，以及不以任何方式循环，甚至是for-in。var theSource = { field1: "TEXT...

问Javascript:仅用一个对象中的现有字段覆盖另一个对象中的字段
EN

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问Javascript:仅用一个对象中的现有字段覆盖另一个对象中的字段EN