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JS Memory Leak School

Retainers and dominators

In a heap snapshot, select an object instance → Retainers tab shows why it’s alive.

Retainer chain example

Section titled “Retainer chain example”
Window / global
  └── leakStore (Array)
        └── (elements)
              └── Object @12345  ← your leaked instance

Read bottom-up: the leaf is the victim; the top is the root keeping it alive.

Dominators

Section titled “Dominators”

Dominator tree — if you removed this object, which other objects would become unreachable?

High retained size + small shallow size = likely collection holding many children (array of DOM nodes, cache Map).

flowchart BT
  obj[Leaked Object]
  arr[Array cache]
  global[global]
  obj --> arr
  arr --> global

Shallow vs retained

Section titled “Shallow vs retained”
Term Meaning
Shallow size Object’s own bytes
Retained size Bytes freed if object is removed

Sort by Retained size to find the biggest wins.

Practical tip

Section titled “Practical tip”

Search snapshot for your domain object (UserSession, ChartWidget). One retained instance is normal; monotonic instance count under repeated actions is the leak.