A detailed description of working with JET elements and classes in your typescript project can be found at:
JET Typescript Usage.
JET Legend Section
The oj-legend-section element is used to declare properties for legend sections and is only valid as the
child of a template element for the sectionTemplate
slot of oj-legend.
<oj-legend data='[[dataProvider]]'>
<template slot='sectionTemplate'>
<oj-legend-section collapsible='on' expanded='[[$current.index == 0 ? "on" : "off"]]'>
</oj-legend-section>
</template>
</oj-legend>