'warning' 事件

• warning <Error> 警告的主要属性是：
  • name <string> 警告的名称。 默认值: 'Warning'。
  • message <string> 系统提供的警告描述。
  • stack <string> 代码中发出警告的位置的堆栈跟踪。

每当 Node.js 触发进程警告时，则会触发 'warning' 事件。

进程警告类似于错误，因为其描述了引起用户注意的异常情况。 但是，警告不是正常 Node.js 和 JavaScript 错误处理流程的一部分。 Node.js 可以在检测到可能导致次优应用程序性能、错误或安全漏洞的不良编码实践时触发警告。

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);   // 打印堆栈跟踪
});

默认情况下，Node.js 会将进程警告打印到 stderr。 --no-warnings 命令行选项可用于抑制默认控制台输出，但 'warning' 事件仍将由 process 对象触发。

以下示例说明了在向事件添加过多监听器时打印到 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

相比之下，以下示例关闭默认警告输出并向 'warning' 事件添加自定义句柄：

$ 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 命令行选项可用于使警告的默认控制台输出包括警告的完整堆栈跟踪。

• 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'.