how to get array row from multidimentional array from id javascript code example

Example 1: javascript define multidimensional array

var iMax = 20;
var jMax = 10;
var f = new Array();

for (i=0;i<iMax;i++) {
 f[i]=new Array();
 for (j=0;j<jMax;j++) {
  f[i][j]=0;
 }
}

Example 2: multi dimensional array javascript

//2D array
let activities = [
    ['Work', 9],
    ['Eat', 1],
    ['Commute', 2],
    ['Play Game', 1],
    ['Sleep', 7]
];