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Cross-frame variable reference problem in using 'with' statement in js_javascript skills
Cross-frame variable reference problem in using 'with' statement in js_javascript skills
Using the with statement during script programming can greatly reduce the length of lines of code using multi-level object references. This is a Syntax Sugar provided to us by JScript. Of course, instead of using with, the same effect can be achieved by using temporary variables. However, naming a variable when creating it is really not a very fancy thing. Even the higher version of MASM also provides an @@ (and @F and @B) to deal with the problem of temporary labels. It seems that everyone is too lazy. .
I also like to use the with statement. It can indeed save a lot of work and make the code clearer, especially when dealing with cross-frame variable references, it is more convenient. Didn't I hit the gun again just for convenience today? When I was getting cross-frame variables in with, I encountered an inexplicable bug and was depressed for a long time.
The code I wrote is actually not complicated. In a with, after doing a lot of variable references and calculation operations in another frame, the result is assigned to a local variable of the function on this page. The example code is roughly as follows: // ...
var hierarchies = [];
with(parent.frames['xxxFrame'])
{
// Generated by referencing the variables and values in xxxFrame What I want
hierarchies.push(xxx);
}
// Start using hierarchies
At this time, hierarchies are going crazy. No matter how you get the value inside, it’s always the same as what you want. It's worth it, but it's not completely wrong (I'll talk about why this is happening later, and this is also a place that seriously confused me when debugging, and I suffered terribly).
Why is this? Finally, I found out that there is a variable called hierarchies in xxxFrame, and it is also an array. The content is similar to what I put, so I said before why I was confused. And that's not all. Because I wrote a similar with to get data across pages before, and it worked very well, I subconsciously felt that the hierarchies in the with were the hierarchies outside the with. I don’t remember whether the variable in xxxFrame was defined by myself. Anyway, I was depressed for a while when debugging:(.
The JScript manual says that the function of with(objects) statements is: to set the statement Default object. However, within with, there is a search rule for the scope of variables, which is to give priority to matching in default objects (objects). Although sometimes it is felt that the scope of default objects is further "distant", like cross-frame. Situation. When the default object reference cannot match, it will match the variables declared before.
This article can only be included in the "Waterloo in debug".
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