JS: Array.prototype.fill

By Xah Lee. Date: . Last updated: .

New in JS2015.

arrayX.fill(value)
  • Modify the array by giving a value to all indexes, including missing indexes in Sparse Array.
  • Return the modified arrayX.
// using fill to fill sparse array
const xx = [];
xx.length = 4;
xx.fill(0);
console.log(xx);
// [ 0, 0, 0, 0 ]

// using fill to change all values of a normal array
const xx = [0, 1, 2];
console.log(xx.fill(7));
// [ 7, 7, 7 ]
console.log(xx);
// [ 7, 7, 7 ]
arrayX.fill(value, startIndex)

Fill start from startIndex 

console.log([0, 1, 2, 3].fill(9, 0));
// [ 9, 9, 9, 9 ]
console.log([0, 1, 2, 3].fill(9, 1));
// [ 0, 9, 9, 9 ]
console.log([0, 1, 2, 3].fill(9, 2));
// [ 0, 1, 9, 9 ]
console.log([0, 1, 2, 3].fill(9, 3));
// [ 0, 1, 2, 9 ]
console.log([0, 1, 2, 3].fill(9, 4));
// [ 0, 1, 2, 3 ]
arrayX.fill(value, startIndex, endIndex)

Replace up to endIndex, not including it. 

console.log([0, 1, 2, 3].fill(9, 0, 0));
// [ 0, 1, 2, 3 ]
console.log([0, 1, 2, 3].fill(9, 0, 1));
// [ 9, 1, 2, 3 ]
console.log([0, 1, 2, 3].fill(9, 0, 2));
// [ 9, 9, 2, 3 ]
console.log([0, 1, 2, 3].fill(9, 0, 3));
// [ 9, 9, 9, 3 ]
console.log([0, 1, 2, 3].fill(9, 0, 4));
// [ 9, 9, 9, 9 ]