In case someone in the future happens upon this, here was my final code. The index.xxx variables were declared elsewhere:
fd.spRendered(() => {
const tabs = fd.container("Tabs1").tabs;
let tabsTitles = ;
tabs.map((tab) => tabsTitles.push(tab._props.title));
indexTraceability = tabsTitles.indexOf("Traceability");
indexAttachments = tabsTitles.indexOf("Attachments");
indexTraining = tabsTitles.indexOf("Training");
indexControl = tabsTitles.indexOf("Control");
});
Thanks again Stepan.