Get the Length of a Buffer in Node.js
Aug 19, 2020
Node.js buffers are objects that store arbitrary binary data. Buffers have a
length property that contains the number of bytes
in the buffer.
const buf = Buffer.from('Hello, World', 'utf8'); buf.length; // 12, same as 'Hello, World'.length
For buffers containing UTF8 encoded strings, the buffer's length is equivalent to the length of the string. For
example, if you read a text file from the file system using
fs, the resulting buffer's length is the same as
the number of characters in the text file.
const fs = require('fs'); fs.writeFileSync('./test.txt', 'Hello, World'); const buf = fs.readFileSync('./test.txt'); Buffer.isBuffer(buf); // true buf.length; // 12, same as 'Hello, World'.length
Allocated vs Actual
Buffer#length contains the number of allocated bytes for the buffer, not how many bytes are
actually used. Often these two are equivalent, but they may be different.
For example, if you allocate a 100-byte buffer using
Buffer.alloc(), the buffer's length will always be 100, regardless
of the buffer's contents.
const buf = Buffer.alloc(100); buf.length; // 100, even though the buffer contains 100 `0` bytes // Write 'Hello, World' to the buffer starting at offset 0 buf.write('Hello, World', 0); buf.length; // still 100, because there's 100 bytes allocated.