问题描述
我有一个包含一些条件的字符串，例如：
I have a string containing some conditions, for example:
var str =this.demoModel.active =='1'& ;& this.demoModel.span> 5 || ...
是否有直接的方法在javascript中解析它们他们的工作就像一系列条件。类似于：
Is there a direct way in javascript to parse them so that, they work like a set of conditions. Something like:
if（JSON.parse（str）{}）。 ??
推荐答案
一般来说，你应该尽量避免陷入这种情况：将JavaScript存储在字符串中供以后评估应该是尽可能避免。根据您的实际情况，您可以考虑以下选项：
In general you should try to avoid getting into this situation at all: storing JavaScript in strings for later evaluation should be avoided if possible. Depending on your actual case you could consider the following option:
它们在实际使用中受到限制，因为它们与使用它们的脚本一起解析，但：
They are limited in practical use, since they are parsed together with the script in which they are used, but they are also safe:
var str = `${this.demoModel.active == '1' && this.demoModel.span > 5}`;
分配此字符串后，它会立即评估 $ {} 其中的部分。
When this string is assigned, it will immediately evaluate the ${ } parts in it.
因此，如果评估可以立即发生，这只是一个解决方案，因为您无法将其存储在字符串中然后期望稍后触发评估。
This is thus only a solution if the evaluation can happen immediately, because you cannot store this in a string and then expect to trigger the evaluation later.
因此与以下内容没有太大区别：
And so it is not much different from:
var bool = this.demoModel.active == '1' && this.demoModel.span > 5;
2。通过回调延迟评估
解决方法可能是定义一个评估模板文字或表达式的函数，如下所示：
2. Deferred evaluation through callback
A work-around could be to define a function that evaluates the template literal or expression, like so:
var rule = function() {
return this.demoModel.active == '1' && this.demoModel.span > 5;
};
...你可以传递该函数，例如作为回调：
... and you can pass that function around, for instance as a callback:
doSomething(rule);
...然后 doSomething 可以像这样调用它，绑定上下文，以便此具有适当的值：
... and then doSomething could call it like this, binding the context, so that this has the appropriate value:
function doSomething(rule) {
if (rule.call(this)) console.log('rule passed');
}
3。嵌套对象数据结构
另一种选择是为表达式创建一个对象结构，例如：
3. Nested Object Data Structure
Another option would be to create an object structure for the expressions, for example like this:
var rules = [
[{ // AND conditions:
"field": "active",
"compare": "eq",
"target": 1
}, {
"field": "span",
"compare": "gt",
"target": 5
}], // Next array will be OR'ed
[{
"field": "...."
"compare": "..",
"target": ...
}]
}];
这是一个嵌套数组，其中顶层将具有必须一起进行OR运算的规则，而内部级别将与AND在一起。
This is a nested array, where the top level will have rules that must be OR'ed together, while the inner level will be AND'ed together.
然后编写一个将处理此结构的函数。 compare 名称可以映射到代码中的函数，如下所示：
Then write a function that will process this structure. The compare names could be mapped to functions in your code, like this:
const comparators = {
"eq": (a, b) = a === b,
"gt": (a, b) = a > b
};
因此，要评估规则数组中的一个对象，可以使用此：
So to evaluate one object in the rules array, you could use this:
execute: (obj) => comparators[this.demoModel[obj.compare]] // get the function
(this.demoModel[obj.field], obj.target) // pass arguments
这个规则结构可以很容易地保存并加载为JSON字符串。
This rules structure can easily be saved and loaded as JSON string.
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