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How to implement Array.prototype.fill? JavaScript Interview Question | Problem Solving | JavaScript Polyfills

@Devtools Tech

In this question, the candidate needs to implement a function customFill that mimics the behaviour of Array.prototype.fill method.

More about Array.prototype.fill

The fill() method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array.length). It returns the modified array.


const array = [1, 2, 3, 4];

// Fill with 0 from position 2 until position 4
console.log(array.fill(0, 2, 4));
// expected output: Array [1, 2, 0, 0]

// Fill with 5 from position 1
console.log(array.fill(5, 1));
// expected output: Array [1, 5, 5, 5]

// expected output: Array [6, 6, 6, 6]


fill(value, start)
fill(value, start, end)


  1. value

Value to fill the array with.

Note: all elements in the array will be this exact value: if value is an object, each slot in the array will reference that object.

  1. start (Optional)

Zero-based index at which to start filling, converted to an integer.

  • Negative index counts back from the end of the array — if start < 0, start + array.length is used.
  • If start < -array.length or start is omitted, 0 is used.
  • If start >= array.length, no index is filled.
  1. end (Optional)

Zero-based index at which to end filling, converted to an integer. fill() fills up to but not including end.

  • Negative index counts back from the end of the array — if end < 0, end + array.length is used.
  • If end < -array.length, 0 is used.
  • If end >= array.length or end is omitted, array.length is used, causing all indices until the end to be filled.
  • If end is positioned before or at start after normalization, no index is filled.

Return value

The modified array, filled with value.


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