What is fill() in Node.js Buffer Module?

The `fill` function in Node.js Buffer Module

The fill function is a member function of Buffer class in the Node.js Buffer Module. The syntax of the fill function is as follows:

buf.fill(value[,offset[,end]][,encoding])

Parameters

value is the value that is used to fill the buffer. It can be any of the following types:
- string
- Buffer
- Uint8Array
- integer
offset is an integer that specifies the number of bytes to skip from the start of the buffer when filling it. The default value is 0.
end is an integer that specifies up to which byte the buffer must be filled (exclusive). The default value is buf.length.
encoding is a string that specifies the encoding type for the value parameter if it is a string. The default value is utf-8.

Return value

Returns a reference to buf of type Buffer.

Description

The fill function fills the buffer buf with the specified value.

Example usage of the `fill` function

The following code snippet provides various examples on how to use the fill function:

const buffers = []
// Create and store 5 buffers with length 10 and fill with 'x's
for (let i = 0; i < 5; i++) {
    buffers.push(Buffer.alloc(10, 'x'));
}
buffers[0].fill('a');
buffers[1].fill('b', 5);
buffers[2].fill('c', 5, 9);
buffers[3].fill('dd', 'hex');
buffers[4].fill('e', 5, 8, 'utf-8');
for (let i = 0; i < 3; i++) {
    console.log(`Buffer[${i}] as String:`);
    console.log(buffers[i].toString());
}
console.log('Buffer[3]:')
console.log(buffers[3]);
console.log('Buffers[4] as String:');
console.log(buffers[4].toString());

In the above example, the array buffers is initialized with 5 Buffer instances of length 10 that are filled with x. buffers[0].fill is called with only the value argument and results in the whole buffer being filled with a. buffers[1].fill is supplied the offset value 5, so in the output, bs occurred at and after the index 5 of the buffer, keeping the first 5 bytes unchanged. buffers[2].fill is supplied offset and end, which have the values 5 and 9 respectively.

Note that c was filled up to the index 9 of the buffer (excluding index 9 itself).

buffers[3].fill is called with value and encoding arguments, which have the values dd and hex respectively. The string dd, if interpreted as a utf-8 string, yields two bytes, one for each d. However, in this case, dd is interpreted as a hex string; since a single digit in hex is equivalent to half a byte, the string dd yields a single byte. buffer[3] is printed as a buffer which shows the values in hexadecimal notation, where 2 characters separated by a space represent a single byte. buffers[4].fill is supplied all parameters; value, offset, end, and encoding. The value e is filled starting from the index 5 byte and is filled up to the index 8 byte (exclusive).

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