'warning' 事件

• warning <Error> 警告的主要属性有：
  • name <string> 警告的名称。默认值: 'Warning'。
  • message <string> 系统提供的对此警告的描述。
  • stack <string> 当警告触发时，包含代码位置的堆栈信息。

任何时候 Node.js 触发进程警告，都会触发 'warning' 事件。

进程警告与进程错误的相似之处，在于两者都描述了需要引起用户注意的异常条件。 区别在于，警告不是 Node.js 和 Javascript 错误处理流程的正式组成部分。 一旦探测到可能导致应用性能问题，缺陷或安全隐患相关的代码实践，Node.js 就可发出警告。

process.on('warning', (warning) => {
  console.warn(warning.name);    // 打印警告名称
  console.warn(warning.message); // 打印警告信息
  console.warn(warning.stack);   // 打印堆栈信息
});

默认Node.js会打印进程警告到stderr。使用--no-warnings的命令行选项可以阻止默认从console输出信息， 但是'warning'事件仍然会被process对象发出。

下面的例子展示了当一个事件绑定了太多的监听器时，输出到stderr的警告。

$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit

与上述相反，如下例子关闭了默认的警告输出，并且给'warning'事件添加了一个定制的处理器。

$ node --no-warnings
> const p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!

--trace-warnings命令行选项可以让默认的控制台输出警告信息时，包含警告的全部堆栈信息。

使用--throw-deprecation命令行选项标志启动Node.js，会使得自定义的弃用警告作为异常信息抛出来。

使用--trace-deprecation命令行选项标志，会使得自定义的弃用警告打印到stderr，包括其堆栈信息。

使用--no-deprecation命令行选项标志，会阻止报告所有的自定义的弃用警告。

*-deprecation 命令行选项标志，只会影响使用名字为 'DeprecationWarning' 的警告。

• warning <Error> Key properties of the warning are:
  • name <string> The name of the warning. Default: 'Warning'.
  • message <string> A system-provided description of the warning.
  • stack <string> A stack trace to the location in the code where the warning was issued.

The 'warning' event is emitted whenever Node.js emits a process warning.

A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs, or security vulnerabilities.

process.on('warning', (warning) => {
  console.warn(warning.name);    // Print the warning name
  console.warn(warning.message); // Print the warning message
  console.warn(warning.stack);   // Print the stack trace
});

By default, Node.js will print process warnings to stderr. The --no-warnings command-line option can be used to suppress the default console output but the 'warning' event will still be emitted by the process object.

The following example illustrates the warning that is printed to stderr when too many listeners have been added to an event:

$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit

In contrast, the following example turns off the default warning output and adds a custom handler to the 'warning' event:

$ node --no-warnings
> const p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!

The --trace-warnings command-line option can be used to have the default console output for warnings include the full stack trace of the warning.

Launching Node.js using the --throw-deprecation command-line flag will cause custom deprecation warnings to be thrown as exceptions.

Using the --trace-deprecation command-line flag will cause the custom deprecation to be printed to stderr along with the stack trace.

Using the --no-deprecation command-line flag will suppress all reporting of the custom deprecation.

The *-deprecation command-line flags only affect warnings that use the name 'DeprecationWarning'.