JavaScript Exception Handling _ javascript tips-js tutorial

This article mainly introduces how to handle JavaScript exceptions. For more information, see the front-end engineers who know that JavaScript has basic exception handling capabilities. We can throw new Error (), and the browser will throw an exception when we call an API Error. However, it is estimated that the vast majority of front-end engineers have not considered collecting such abnormal information.

Anyway, as long as the refresh does not recur after a JavaScript error occurs, the user can solve the problem through refresh, And the browser will not crash. If it has not happened. This assumption was established before the Single Page App became popular. After the Single Page App is running for a period of time, the status is extremely complex. You may have entered a number of operations before coming here. Do you just need to refresh the Page? Shouldn't the previous operations be completely redone? Therefore, it is necessary to capture and analyze the exception information, and then we can modify the code to avoid affecting the user experience.

Capture exceptions

The throw new Error () We write ourselves can be captured, because we know exactly where the throw is written. However, exceptions that occur when you call the browser API are not so easy to capture. Some APIs throw exceptions in the standard, and some APIs throw exceptions only in some browsers due to implementation differences or defects. For the former, we can also capture through try-catch. For the latter, we must listen to global exceptions and then capture them.

Try-catch

If some browser APIs are known to throw an exception, we need to put the call in try-catch to prevent the entire program from entering the invalid state due to an error. For example, window. localStorage is an API that throws an exception when the write data exceeds the capacity limit. This is also true in Safari's private browser mode.

The Code is as follows:

Try {
LocalStorage. setItem ('date', date. now ());
} Catch (error ){
ReportError (error );
}

Another common use case of try-catch is callback. Because the code of the callback function is uncontrollable, we do not know the code quality and whether to call other APIs that will throw exceptions. In order not to make other code after callback unable to be executed because of a callback error, it is necessary to put the call back into try-catch.

The Code is as follows:

Listeners. forEach (function (listener ){
Try {
Listener ();
} Catch (error ){
ReportError (error );
}
});

Window. onerror

If an exception occurs, try-catch can only be captured through window. onerror.

The Code is as follows:

Window. onerror =
Function (errorMessage, scriptURI, lineNumber ){
ReportError ({
Message: errorMessage,
Script: scriptURI,
Line: lineNumber
});
}

Be careful not to use window. addEventListener or window. attachEvent to listen for window. onerror. Many browsers only implement window. onerror, or only the implementation of window. onerror is standard. Considering that the standard draft defines window. onerror, we can use window. onerror.

Attribute loss

Suppose we have a reportError function to collect caught exceptions and send them to the server for storage in batches for query and analysis. What information do we want to collect? Useful information includes: Error Type (name), error message, script, line, and column) stack ). If an exception is captured through try-catch, the information is on the Error object (supported by mainstream browsers), so reportError can also collect the information. However, if it is captured through window. onerror, we all know that this event function has only three parameters, so the unexpected information of these three parameters is lost.

Serialize messages

If the Error object is created by ourselves, the error. message is controlled by us. Basically, what we put in error. message is what the first parameter of window. onerror is. (The browser will actually slightly modify it, for example, adding the 'uncaught Error: 'prefix .) Therefore, we can serialize the attributes we are concerned with (for example, JSON. Stringify) and store them in error. message. Then we can read window. onerror and deserialize it. Of course, this is limited to Error objects created by ourselves.

Fifth Parameter

Browser vendors also know the restrictions imposed on the use of window. onerror, so they began to add new parameters to window. onerror. Considering that only row numbers without column numbers do not seem very symmetric, IE first adds the column numbers and places them in the fourth parameter. However, you are more concerned about whether you can obtain the complete stack. So Firefox should put the stack in the fifth parameter. But Chrome said it would be better to put the entire Error object in the fifth parameter. You can read any attribute, including custom attributes. As a result, Chrome is fast, and a new window. onerror signature is implemented in Chrome 30, the standard draft is written like this.

The Code is as follows:

Window. onerror = function (
ErrorMessage,
ScriptURI,
LineNumber,
ColumnNumber,
Error
){
If (error ){
ReportError (error );
} Else {
ReportError ({
Message: errorMessage,
Script: scriptURI,
Line: lineNumber,
Column: columnNumber
});
}
}

Attribute Normalization

The names of the Error object attributes we discussed earlier are all based on the Chrome naming method. However, the names of Error object attributes vary in different browsers, for example, the script file address is called script in Chrome, but filename in Firefox. Therefore, we also need a special function to normalize the Error object, that is, to map different attribute names to uniform attribute names. For details, refer to this article. Although browser implementations are updated, it is not difficult to manually maintain such a ing table.

Similar to stack tracing. This attribute saves the stack information when an exception occurs in plain text. Because the text format used by each browser is different, you also need to manually maintain a regular expression, extract the identifier, script, line, and column numbers of each frame from plain text ).

Security restrictions

If you have also encountered an error with the message 'script error. ', you will understand what I am talking about. This is actually a browser's limitation on different origin Script files. The reason for this security restriction is as follows: assume that the HTML returned by an online banking user after logon is different from the HTML displayed by an anonymous user, a third-party website can put the website's URI in the script. in the src attribute. HTML cannot be parsed as JS, so the browser throws an exception, and this third-party website can determine whether the user has logged on by parsing the exception location. For this reason, the browser filters all exceptions thrown by different source Script files. Only the unchanged message 'script error. 'is filtered out, and all other attributes disappear.

For websites of a certain scale, it is normal that script files are stored on the CDN, with different origins. Now, even if you build a small website, common frameworks such as jQuery and Backbone can directly reference versions on the public CDN to accelerate user download. Therefore, this security restriction does cause some trouble. As a result, the exception information we collect from Chrome and Firefox is useless 'script error .'.

CORS

To bypass this restriction, you only need to ensure that the script file and the page itself are the same source. However, if you place a script file on a server without CDN acceleration, will the download speed be reduced? One solution is to store the script file on CDN, use XMLHttpRequest to download the content through CORS, and then create a script tag to inject it into the page. The embedded code on the page is of course the same source.

This is easy to say, but there are many details to implement. In a simple example:

The Code is as follows:

Script
(Function step2 (){})();
Script

We all know that if this step 1, step 2, and step 3 have dependencies, they must be executed strictly in this order; otherwise, errors may occur. The browser can request files in step 1 and Step 3 in parallel, but the execution sequence is guaranteed. If we use XMLHttpRequest to retrieve the file content of step 1 and step 3, we need to ensure that the order is correct. In addition, do not forget step 2. When Step 1 is downloaded in non-blocking form, step 2 can be executed. Therefore, we must manually intervene in step 2 and wait until step 1 is complete before execution.

If we already have a set of tools to generate the script tag on different pages on the website, we need to adjust this tool to make changes to the script Tag:

The Code is as follows:

Script
ScheduleRemoteScript ('HTTP: // cdn.com/step1.js ');
Script
Script
ScheduleInlineScript (function code (){
(Function step2 (){})();
});
Script
Script
ScheduleRemoteScript ('HTTP: // cdn.com/step3.js ');
Script

We need to implement the scheduleRemoteScript and scheduleInlineScript functions, and ensure that they are defined before the first script tag that references the external script file, then the remaining script tags will be rewritten to the above form. Note that the originally executed step2 function is placed in a larger code function. The code function will not be executed. It is just a container, so that the original step2 code can be saved without escaping, but will not be executed immediately.

Next, we need to implement a complete mechanism to ensure that the file content downloaded by scheduleRemoteScript Based on the address and the Code directly obtained by scheduleInlineScript can be executed one by one in the correct order. The detailed code is not provided here. If you are interested, you can implement it yourself.

Row number lookup

Getting content through CORS and injecting code into the page can break through security restrictions, but it introduces a new problem, that is, line number conflict. Originally, error. script can be used to locate a unique script file, and then error. line can be used to locate a unique row number. Because the code is embedded on the page, multiple script labels cannot pass error. script. However, the line numbers in each script label are counted from 1, so we cannot use the exception information to locate the source code location where the error is located.

To avoid line number conflicts, we can waste some line numbers so that the line numbers used by the actual code in each script tag do not overlap with each other. For example, if no more than 1000 lines of actual code exist in each script tag, I can occupy 1st-1000 lines of code in the first script tag, let the code in the Second script label occupy 1,001st-2000 rows (the first 1000 rows of blank lines are inserted ), the code of the third script tag occupies 2001st-3000 rows (the first 2000 rows are inserted blank lines), and so on. Then we use the data-* attribute to record the information for reverse query.

The Code is as follows:

Data-src = "http://cdn.com/step1.js"
Data-line-start = "1"
>
// Code for step 1
Script