Simplifying IF/ELSE condition

I want to ask how can I simplify my code? It seems hard to read and has too much if-else condition here. Any way to simplify the code?

if (e.shiftKey && this.idx > 0) {
  this.idx= this.idx - 1;
} else if (!e.shiftKey && this.idx < trapFocus.length - 1) {
  this.idx = this.idx + 1;
} else if (!e.shiftKey && this.idx < trapFocus.length + 1) {
  this.idx= this.idx - 2;
} else if (e.shiftKey && this.idx > - 1) {
  this.idx= this.idx + 2;
}