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blog-gist-2019-7-array-fill-0.js

// Initialize a 3x4 matrix with "false" values.
const matrix = Array(3).fill(Array(4).fill(false));

// some stringify to capture the actual values at this point of the execution
console.log(JSON.stringify(matrix));

// set middle value to true
matrix[1][1] = true;

console.log(JSON.stringify(matrix));

blog-gist-2019-7-array-fill-1.js

[[false,false,false,false],
 [false,false,false,false],
 [false,false,false,false]]

[[false,true,false,false],
 [false,true,false,false],
 [false,true,false,false]]

blog-gist-2019-7-array-fill-2.js

const B = Array(4).fill(false);
const A = Array(3).fill(B);
const matrix = A;

blog-gist-2019-7-array-fill-3.js

const matrix = Array(3).fill().map(el => Array(4).fill(false));