在 Javascript 中，字典理解，或对象`map`

In Javascript a dictionary comprehension, or an Object `map`

提问by Cuadue

I need to generate a couple of objects from lists in Javascript. In Python, I'd write this:

我需要从 Javascript 中的列表生成几个对象。在 Python 中，我会这样写：

{key_maker(x): val_maker(x) for x in a_list}

Another way to ask is does there exist something like jQuery.map()which aggregates objects? Here's my guess (doesn't work):

另一种提问方式是是否存在jQuery.map()聚合对象之类的东西？这是我的猜测（不起作用）：

var result = {}
$.map(a_list, function(x) {
    $.extend(result, {key_maker(x): val_maker(x)})
})

采纳答案by Cuadue

Assuming a_listis an Array, the closest would probably be to use .reduce().

假设a_list是一个数组，最接近的可能是使用.reduce().

var result = a_list.reduce(function(obj, x) {
    obj[key_maker(x)] = val_maker(x);
    return obj;
}, {});

Array comprehensions are likely coming in a future version of JavaScript.

数组推导式可能会出现在 JavaScript 的未来版本中。

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You can patch non ES5 compliant implementations with the compatibility patch from MDN.

您可以使用MDN 中的兼容性补丁修补非 ES5 兼容的实现。

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If a_listis not an Array, but a plain object, you can use Object.keys()to perform the same operation.

如果a_list不是数组，而是普通对象，则可以使用它Object.keys()来执行相同的操作。

var result = Object.keys(a_list).reduce(function(obj, x) {
    obj[key_maker(a_list[x])] = val_maker(a_list[x]);
    return obj;
}, {});

回答by fizzyh2o

Here's a version that doesn't use reduce:

这是一个不使用reduce的版本：

Object.fromEntries( a_list.map( x => [key_maker(x), value_maker(x)]) );

Object.fromEntriesis basically the same as _.fromPairsin Lodash. This feels the most like the Python dict comprehension to me.

Object.fromEntries与Lodash中的 _.fromPairs基本相同。这对我来说最像 Python dict 理解。

回答by benwixen

Old question, but the answer has changed slightly in new versions of Javascript. With ES2015 (ES6) you can achieve a one-liner object comprehension like this:

老问题，但答案在新版本的 Javascript 中略有变化。使用 ES2015 (ES6)，您可以实现这样的单行对象理解：

a_list.reduce((obj, x) => Object.assign(obj, { [key_maker(x)]: value_maker(x) }), {})

回答by Steven Almeroth

ES5introduced Mapfor an OrderedDict. A Map comprehension might look like:

ES5Map为OrderedDict引入。地图理解可能如下所示：

Map( Array.map(function(o){return[ key_maker(o), val_maker(o) ]}))

Example:

例子：

> a_list = [ {x:1}, {x:2}, {x:3} ]
< [ Object, Object, Object ]
>
> let dict = new Map(a_list.map(function(o){return[ o.x, o.x**2 ]}))
< Map[3]
< 0 : {1 => 1}
< 1 : {2 => 4}
< 2 : {3 => 9}
>
> dict.get(2)
< 4

回答by Elias Zamaria

Maybe something like this, using Lodash: var result = _.fromPairs(a_list.map(x => [key_maker(x), value_maker(x)]));

也许像这样，使用Lodash： var result = _.fromPairs(a_list.map(x => [key_maker(x), value_maker(x)]));

回答by smartexpert

A shorter ES6 version would be:

较短的 ES6 版本将是：

a_list.reduce((obj, x) => (key_maker(obj[x]) = val_maker(x), obj),{})