Understand code by seeing it

See what happens inside your code.

Write code. Watch variables change, functions execute, and data transform — one step at a time.

Open Playground Explore Examples
  • Your real code, recorded
  • Runs in your browser
  • No account needed
array-map.jsLive trace

Execution Flow

Step 1 of 15
numbersArray(3)
  1. 2
    0
  2. 4
    1
  3. 6
    2

Declares numbers = [2, 4, 6]

Variable declared numbers = [2, 4, 6]

console ›

Interactive examples

Try it yourself.

Explore real examples and see how code comes to life.

View all examples

Array transformation

See how map, filter and reduce work.

  1. const nums = [1, 2, 3, 4];
  2. const squared = nums.map(n => n * n);
Arraynums
  1. 0
    1
  2. 1
    2
  3. 2
    3
  4. 3
    4
map(n => n * n)
Arraysquared
  1. 0
    1
  2. 1
    4
  3. 2
    9
  4. 3
    16
Open Example

Function calls

Step through function execution.

  1. function add(a, b) {
  2. return a + b;
  3. }
  4. const result = add(5, 3);

Call Stack

  1. add(a, b)

    a = 5, b = 3

  2. (return)

    8

  3. result

    8

Open Example

Objects & references

Understand memory and references.

  1. const user = { name: 'Jana' };
  2. const admin = user;
  4. admin.name = 'Developer';
user
{
  name: 'Developer'
}
admin
{
  name: 'Developer'
}
Same reference
in memory
Open Example

A better way to learn and debug

One workspace.
Three perspectives.

Switch between code, visualizations, and a split view. Everything stays in sync.

  • Split

    Code and visualization side by side.

  • Visual

    Focus on execution and data flow.

  • Code

    A clean editor with inline execution info.

Built for understanding

Runs your real code.
Designed for clarity.

reify instruments your JavaScript or TypeScript, runs it in a sandboxed worker in your browser and records every step, so you can replay it forwards and backwards without running it again.

  • Real execution

    Your code, not a simulation

  • Instant replay

    Scrub without re-running

  • Browser-based

    No installation required

  • Private

    Code never leaves your tab

Ready to explore?

Stop guessing.
Start seeing.

Open the playground and experience code in a whole new way.

Open Playground View on GitHub