New to logging? See the Intro guide to Node.js logging and the Best practices for logging in Node.js.
Not using Logtail yet? Check out the Quick start guide to install and set up the Logtail JavaScript client.
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JavaScript advanced usage
Get started with Logtail in your JavaScript application using this overview of the Logtail logger.
Log levels
Logtail JavaScript client instance provides 4 logging methods for the 4 default log levels. The log levels and their methods are:
- DEBUG - Send debug messages with
debug() - INFO - Keep track of your app's progress with
info() - WARN - Report non-critical issues with
warn() - ERROR - Log serious problems with
error()
Logging example
To send a log message of selected log level, use the corresponding method. In this example, we send the DEBUG level log and ERROR level log:
Send logs to Logtail
// Send DEBUG level log using the debug() method
logtail.debug("I am using Logtail!");
// Send ERROR level log using the error() method
logtail.error("Oops! An runtime ERROR occurred!");
The Logtail client automatically adds context to the logs.
The following JSON output is sent to Logtail:
Output
{
"dt":"2022-02-01 12:01:10.127 UTC",
"context":{
"runtime":{
"column_integer":"21",
"file_string":"node_modules/@logtail/core/dist/cjs/base.js",
"line_integer":"141",
"type_string":"Node",
"function_string":"debug",
"method_string":"debug"
},
"system":{
"main_file_string":"/mnt/d/js_logtail/index.js",
"pid_integer":"4193"
}
},
"level_string":"debug",
"message_string":"I am using Logtail!"
}
{
"dt":"2022-02-01 12:01:10.127 UTC",
"context":{
"runtime":{
"column_integer":"8",
"file_string":"index.js",
"line_integer":"40",
"type_string":"Object"
},
"system":{
"main_file_string":"/mnt/d/js_logtail/index.js",
"pid_integer":"4193"
}
},
"level_string":"error",
"message_string":"Oops! A runtime ERROR occurred!"
}
Logging structured data
Quickly troubleshoot your application by logging structured data. Pass an object as a second argument to any logging method, like in the example below:
Send structured logs to Logtail
logtail.warn("Something is not quite right.", {
user: {
username: "John Doe",
email: "john@example.com"
},
additional_info: {
tried_accessing: "/url/of/error"
}
});
This creates the following JSON output:
Output
{
"dt":"2022-02-01 12:01:10.127 UTC",
"context":{
"runtime":{
"column_integer":"8",
"file_string":"index.js",
"line_integer":"29",
"type_string":"Object"
},
"system":{
"main_file_string":"/mnt/d/js_logtail/index.js",
"pid_integer":"4193"
}
},
"level_string":"warn",
"message_string":"Something is not quite right.",
"additional_info":{
"tried_accessing_string":"/url/of/error"
},
"user":{
"email_string":"someuser@example.com",
"username_string":"someuser"
}
}
Middleware
You can intercept every logged item and modify it before it's pushed to Logtail. This could be useful, for example, for adding the current user's ID to the log or for filtering:
Enrich logs
Filter logs
// Intercept the log and add userId to the content
async function enrichLogs(log) {
return {
...log,
userId: getCurrentUserId()
};
}
// Tell Logtail client to use the enriching function
logtail.use(enrichLogs);// Ignore any log with "[ignore]" in its message
async function filterLogs(log) {
const shouldIgnore = log.message?.includes("[ignore]");
return shouldIgnore ? null : log;
}
// Tell Logtail client to use the filtering function
logtail.use(filterLogs);TypeScript
If you're using Logtail in a TypeScript codebase, you can take advantage of our types. You could write the enriching middleware function with types like this:
Enrich logs in TypeScript
import { ILogtailLog } from "@logtail/types";
async function enrichLogs(log: ILogtailLog): Promise<ILogtailLog> {
return {
...log,
userId: getCurrentUserId()
};
}