The variable leds needs to be defined as an array before it can be used as one. Then you can add elements to it using the push method; and these elements can be other (nested) arrays. For example:
//set leds as an empty array
let leds = [];
//add to the array
leds.push([]);
leds[0] = [1,2,3];
leds.push([]);
leds[1] = [4,5,6];
leds.push([7,8,9]);
//change an array value
leds[1][1] = 0;
console.log(leds);CLICK HERE to find out more related problems solutions.