The read-only detail property returns arbitrary metadata that was included in the mark upon construction (either when using performance.mark() or the PerformanceMark() constructor).

Returns the value it is set to (from markOptions of performance.mark() or the PerformanceMark() constructor).

The following example demonstrates the detail property.

performance.mark("dog", { detail: "labrador" });

const dogEntries = performance.getEntriesByName("dog");

dogEntries[0].detail; // labrador