Expression selector 'thisComp' resolves to master property composition instead of text layer composition in V8 engine

Direct Answer

Under the V8 engine (After Effects 16.0+), thisComp inside a text layer's expression selector resolves to the composition where the Master Property instance is being evaluated — the "child" composition — not the text layer's original host composition. This differs from the legacy ExtendScript behavior and is not formally documented by Adobe.

Confirmed Facts

• ExtendScript (pre-16.0): thisComp binds to the text layer's containing composition at compile time, even when pulled via Master Properties.
• V8 (16.0 through current 24.x): thisComp binds at render time to the composition currently evaluating the expression — the child composition when Master Properties drive the layer.
• No per-expression engine override exists; the project setting File > Project Settings > Expressions > JavaScript Engine applies globally.
• Legacy ExtendScript is deprecated, shows a warning banner in AE 23+, and disables multi-threaded evaluation.

Likely Explanation

The V8 engine evaluates expression selectors during the render pass of the composition currently being previewed or rendered. When a Master Property instance drives a text layer's Source Text, the selector runs in the child composition's context, so thisComp reflects that child. ExtendScript appears to have captured the layer's original host composition at expression-compile time.

Recommended Workaround

Replace thisComp with thisLayer.containingComp (introduced in AE 17.0). This property returns the composition that directly owns the layer, regardless of Master Property context, and behaves identically in both engines.

// Replace this:
thisComp.name

// With this (AE 17.0+):
thisLayer.containingComp.name

For projects that must open in AE 16.x, add a version guard:

var hostComp = thisLayer.containingComp || thisComp;
hostComp.name;

Verification Steps

1. Create a minimal test: MasterComp with a text layer containing an expression selector reading thisComp.name.
2. Create ChildComp, add MasterComp as a layer, expose Source Text as a Master Property.
3. Observe the selector result in both compositions under V8 and (temporarily) ExtendScript.
4. Apply the containingComp workaround and confirm consistent results.
5. Test in the oldest and newest AE versions you support (e.g., 23.6 and 24.5).

One Missing Diagnostic Detail

Does your text layer sit on the source side (inside the master composition being exposed) or the instance side (the layer in the child composition receiving the Master Property override)? thisLayer.containingComp returns the master composition on the source side and the child composition on the instance side — verify which side your selector runs on.