Add dynamic value when writing log message with log4js - node.js

Question

I want to add a dynamic attribute to a pattern I am using with log4js.
I am using some custom pattern, something like this:
"%d{} %-6p[%thread] %c [%x{context}]: %m%n%r"
Context is the dynamic value that I want to set with some unique id generated for each user on the server side.
There is a way to add dynamic value when creation log4js configuration by using "tokens" and "context" attributes.
But in this case values should be set during the log creation.
Is there a way to add some dynamic attribute that is set when the actual message is written to the log and not during the config phase?
Right now I am doing something like this:
log4js.configure(
{
appenders: { "file": { "type": "file", "filename": "service.log", "maxLogSize": 102400, "backups": 5, "category": "com.nextinsurance", "layout": { "type": "pattern", "pattern": "%d{} %-6p[%thread] %c [%x{context}]: %m%n%r", "tokens" : {context: function(logEvent){ return getContextFromData(logEvent) } } } }, "console" : {"type": "console"} },
categories: { "default": { "appenders": ["file", "console"], "level": "info" } }
}
);
But want to inject this value when writing to log, something like
logger.info(Message, {context: context_value})

Answer 1

You can use logEvent data property to fetch context. logEvent data property contains the array of args passed in log event.
Here is the sample code:
var log4js = require("log4js");
log4js.configure({
appenders: {
out: {
type: 'stdout',
layout: {
type: 'pattern',
pattern: '[%d] [%p] [%c] [%x{context}] - %m%n',
tokens: {
context: function(logEvent) {
let returnVal = logEvent.data[1] ? logEvent.data[1].context : undefined;
if (returnVal) {
logEvent.data.pop();
} else {
returnVal = 'null'; // set any default value if context is not passed.
}
return returnVal;
}
}
}
}
},
categories: {
default: {
appenders: ['out'],
level: 'INFO'
}
}
});
log4js.level = 'info';
let logger = log4js.getLogger();
logger.info('Hello', { context: 'context_value'}); // prints [2019-09-13T16:50:48.818] [INFO] [default] [context_value] - Hello
logger.info('Hello'); // prints [2019-09-13T16:50:48.820] [INFO] [default] [null] - Hello