Log levels and when to use them in Express - Step-by-Step Execution

Concept Flow - Log levels and when to use them

Start Logging Event

↓

Determine Log Level

↓

Is Level: Error?

Yes→Log as ERROR

↓

Send Alert if needed

↓

Is Level: Warn?

Yes→Log as WARN

↓

Review Warning

↓

Is Level: Info?

Yes→Log as INFO

↓

General Operation Logs

↓

Is Level: Debug?

Yes→Log as DEBUG

↓

Detailed Dev Info

↓

Is Level: Trace?

Yes→Log as TRACE

↓

Very Fine Detail

↓

End Logging Event

This flow shows how a logging event is checked against log levels from highest severity (Error) to lowest (Trace) to decide how to log it.

Execution Sample

Express

const log = (level, message) => {
  switch(level) {
    case 'error': console.error(message); break;
    case 'warn': console.warn(message); break;
    case 'info': console.info(message); break;
    case 'debug': console.debug(message); break;
    case 'trace': console.log(message); break;
  }
};

This function logs messages to the console based on the log level provided.

Execution Table

Step	Input Level	Message	Action Taken	Console Output
1	error	"Server crashed"	Log as ERROR	Error: Server crashed
2	warn	"Memory usage high"	Log as WARN	Warning: Memory usage high
3	info	"User logged in"	Log as INFO	Info: User logged in
4	debug	"Variable x=10"	Log as DEBUG	Debug: Variable x=10
5	trace	"Function start"	Log as TRACE	Log: Function start
6	verbose	"Extra details"	No matching level, no log

💡 Logging stops after matching the input level or no match found.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
level	undefined	error	warn	info	debug	trace	verbose
message	undefined	Server crashed	Memory usage high	User logged in	Variable x=10	Function start	Extra details

Key Moments - 3 Insights

Why does the 'verbose' level not produce any output?

Because 'verbose' is not handled in the switch cases (see execution_table row 6), so no logging action is taken.

Why do we use different console methods like console.error and console.warn?

Each console method matches the log level severity and helps tools or developers quickly identify the importance of the message (see execution_table rows 1 and 2).

What happens if we pass a log level not defined in the function?

No logging occurs because the switch has no matching case, so the function does nothing (see execution_table row 6).

Visual Quiz - 3 Questions

Look at the execution table, what console method is used at step 3?

A console.info

B console.warn

C console.error

D console.debug

💡 Hint

Check the 'Action Taken' and 'Console Output' columns at step 3.

At which step does the logging function not produce any output?

A Step 4

B Step 6

C Step 2

D Step 1

💡 Hint

Look for the row where 'Console Output' is empty in the execution table.

If we add a 'verbose' case to the switch, what would change in the execution table?

A Step 6 would log a message instead of no output

B Step 5 would stop logging

C Step 1 would log twice

D No change at all

💡 Hint

Consider what happens when a new matching case is added for 'verbose' in the switch.

Concept Snapshot

Log levels help organize messages by importance.
Common levels: error, warn, info, debug, trace.
Use error for serious problems, warn for caution, info for normal events.
Debug and trace show detailed info for developers.
In Express, use console methods matching levels.
Unrecognized levels produce no output.

Full Transcript

This lesson shows how logging works in Express using different log levels. When a message is logged, the code checks its level from error down to trace. Each level uses a different console method to show the message with the right importance. For example, errors use console.error and warnings use console.warn. If the level is not recognized, no message is logged. This helps developers and operators quickly see what is happening and how serious it is. The example function uses a switch statement to pick the right console method. The execution table shows what happens step by step for different levels. Beginners often wonder why some messages don't show up or why different console methods are used. This visual trace clears those doubts by showing exactly what happens inside the function.