Here the basic code used to add a row and to retrieve all the sub fields.
// Get repeater field (replace my-repeater-field-key' with a field key)
repeater = acf.getField('my-repeater-field-key');
// Add a new row
$row = repeater.add();
// Get subfields
sub_fields[i] = acf.getField($el);
// show sub fields
// loop around sub fields and populate them.
Not so tricky with ACF JS API…
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