Introduction: Advanced Logging

Advanced Logging features used together with Value Explorer and Running Selected Tests features enables a highly productive workflow.

There are a number of ways to log something in wallaby.js and quickly inspect the output inline, or inside the Failing Tests/Line tests view:


Logpoints allow VS Code and JetBrains IDE breakpoints to be used to display the value of any expression. Using breakpoints to manage displaying values offers a number of advantages:

  • No modification to source code is necessary
  • They are easily added and removed using familiar keyboard shortcuts (VS Code: F9, or ⇧F9 for inline breakpoints, JetBrains IDEs: ⌘F8)
  • They offer a clear visual indicator of what is being logged (especially when placed within lines as inline breakpoints)
  • No debugger required (Wallaby uses the breakpoint information directly to determine what to log)
  • They are managed by your editor, and will persist when files are closed and reopened
  • They can be added to classes and functions to log all lines within them
Watch logpoints in action

Using console.log

For those new to Wallaby, a familiar method to log values is to simply use console.log(anything). Wallaby supports logging values with the same console.log method that you are used to.

Watch Expressions

Identifier expressions

Another simple way to log something is to just type a variable name and get its value(s) displayed:

Watch expressions

Note that only simple identifier expressions, for example a, are automatically logged this way, but not other types of expressions, for example not a.b or a.b(). To log other types of expressions, you may use one of the other logging methods described in this article.

Live Comments


Live Comments allow special comments to be used to display values. Inserting the special comment /*?*/ after an expression (or just //? after a statement) will log just the value of that expression.

Like Logpoints, Live Comments allow you to see the value right in the middle of an expression. For example, given a chain of a.b().c().d(), you may want to inspect the result of a.b().c() before .d() is called.

The example below shows how to log the runtime value of a.b().c():


If you want to log the full expression of a.b().c().d() then you can add a comment at the end of the expression:

a.b().c().d() /*?*/
// or just
a.b().c().d() //?

You may also write any JavaScript code right in the comment to shape the output. The code has the access to the $ variable which is the expression that the comment is appended to. The executed code is within a closure, so it also has the access to any objects that you may access from within the current lexical environment.

Watch expressions in action

Note that there’s no constraints in terms of what the comment code can do. For example, the watch comment below is incrementing d.e value, and returning $, which points to the expression that the comment is appended to (a.b).

Logpoints, Live Comments, and Live Value Display (but not console.log) also have some built in smarts to help with logging certain data types. For example, when you place the comment after an expression that is a promise, the resolved value of the promise is logged. Or, if an expression is an observable, then its values are displayed.

Promises and Observables

Note that the special logging comment feature (as well as the Performance Testing one) will not work in most cases for CoffeeScript code due to the CoffeeScript compiler limitations.

Live comment snippet

To save some time on typing the comment when you need it, you may:

Object proxy

The feature allows you to wrap any selected object value with JavaScript Proxy to quickly see where the wrapped object properties are being accessed from and what values are returned/set, and also where the wrapped object functions are called from and what values are returned from the object functions.

Inserting the special comment //?? (or /*??*/) after any expression will wrap the runtime value of the expression with recursive proxy (if the value of the expression is an object). The results of intercepting the object’s properties and functions can be viewed in Wallaby Value Explorer.

Value Explorer accessed members

Depending on how the wrapped object is used in your code, the following nodes can be displayed under the Accessed Members node of the object’s value in Wallaby Value Explorer:

  • property read node with GET label and the value of the property being read,
  • property write node with SET label and the value being assigned to the property,
  • property delete node with DELETE label and the value of the property when it was deleted,
  • function call node with CALL label and the value returned from the function.

Under each intercepted property/function node there is a Call Stack node that can be used to find out where exactly in your code the property was accessed from or the function was called from.

Please note that in order for an object’s properties and functions to be intercepted, the object proxy live comment must be applied to an identifier prior to its use (e.g. variable declaration, method parameter). Applying the object proxy to an identifier expression alone will not intercept subsequent usages of the object.

  // this will intercept all accessed members of "myInterceptObject"
  const myInterceptedObject = getComplexObject(); //??

  // this will intercept all accessed members of "myComplexObject"
  // within the function "interceptUsagesInThisFunction"

  // this will NOT intercept any accessed members
  const myInterceptedObject = getComplexObject();
  getComplexObject(); //??

Performance Testing

The feature also allows you to quickly see how various parts of your code perform. It can be very helpful for identifying possible bottlenecks in your app and for doing the performance optimization.

Live Performance Testing in action

Inserting the special comment /*?.*/ after any expression will report how much time it took to execute the expression.

Live Performance Testing in action

Adding the comment to an expression that gets executed multiple times, for example inside a loop, will make the tool to display total execution time, average execution time (total execution time divided by number of times the expression had been executed), minimum and maximum execution time.

Live Performance Testing in action

Execution time and result via a single comment

You may also use the execution time reporting comment and add to it any expression that you may use in live comments to display both an expression execution time and execution result.

For example,

a() //?. $

displays a() execution time and result.


The watch comment prefix default value (?) can be changed in wallaby config file. For example, to make the watch comment to be /*out:*/, you may do:

module.exports = function (wallaby) {
  return {

    hints: {
      commentAutoLog: 'out:'

Live Value Display

Live Comments feature provides an excellent way to log any expression value and to keep displaying the value when you change your code.

Sometimes you may also want to quickly display some expression value, but without modifying your code. Live Value Display allows you to do it with a special command (Show Value command/intention action, or with a corresponding keyboard shortcut).

Note that the expression being logged either needs to be selected, or the cursor position needs to be right after the expression being logged when the command is invoked. It works just as if you were inserting a live comment where your cursor is located.

Show Value / Copy Value / Show Timing(s)

Wallaby provides two commands that allow you to quickly get the value of an expression without changing your code:

  • Show Value displays the value of your expression in your code editor right beside the expression, and will allow you to explore the value using wallaby’s Value Explorer.
  • Copy Value displays the value of your expression in your code editor right beside the expression AND expands the value and copies the value to the clipboard. Like Copy Value, you may explore the value using wallaby’s Value Explorer.
  • Show Line Value(s) is available for VS Code and JetBrains editors and will output the same as Show Value for the selected line(s) as if you had selected the entire line to display.
  • Show Line Timing(s) is available for VS Code and JetBrains editors and will output Execution time for the selected line(s) as if you had used the //?. comment for the line.

Please note: Copy Value expansion has the same limitations as Wallaby’s Value Explorer Auto-Expand feature.

Value Explorer Integration

Wallaby’s Value Explorer feature allows viewing and navigating objects logged as console.log, identifier expressions, live comments, and the Show Value command.

Identifier expressions and Live Comments can be provided with an additional hint to automatically expand objects when they are logged to Value Explorer. Inserting the special comment //?+ after any expression will expand your object and its properties within the Value Explorer tree.

Value Explorer Auto-Expand

Note that automatically expanded objects have the following limitations:

  • Cyclic Dependencies are not automatically expanded
  • Functions are not automatically expanded
  • Strings beyond 8192 characters are not automatically expanded
  • Only the first 100 properties on a single object will be expanded
  • Only the first 100 elements of an array will be expanded
  • Only the first 10 levels of nested properties will be expanded
  • Only the first 5000 properties across all objects will be expanded

Automatically expanded objects also resolve property getters, so in case of

var a = {
  get b() {
    return 1;

a; //?+

the output will be { b: 1 }.

Resolve property getters

Automatically expanded objects (logged with the special comment //?+) resolve property getters. If you want to always resolve property getters regardless whether the logged object is automatically expanded or not, you may use the resolveGetters configuration setting.