Node.js v8.17.0 Documentation

Table of Contents

Buffer#

Stability: 2 - Stable

Prior to the introduction of TypedArray, the JavaScript language had no mechanism for reading or manipulating streams of binary data. The Buffer class was introduced as part of the Node.js API to enable interaction with octet streams in TCP streams, file system operations, and other contexts.

With TypedArray now available, the Buffer class implements the Uint8Array API in a manner that is more optimized and suitable for Node.js.

Instances of the Buffer class are similar to arrays of integers but correspond to fixed-sized, raw memory allocations outside the V8 heap. The size of the Buffer is established when it is created and cannot be changed.

The Buffer class is within the global scope, making it unlikely that one would need to ever use require('buffer').Buffer.

Examples:

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10, filled with 0x1.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using either fill() or write().
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing [0x1, 0x2, 0x3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing UTF-8 bytes [0x74, 0xc3, 0xa9, 0x73, 0x74].
const buf5 = Buffer.from('tést');

// Creates a Buffer containing Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf6 = Buffer.from('tést', 'latin1');

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

In versions of Node.js prior to v6, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided:

  • Passing a number as the first argument to Buffer() (e.g. new Buffer(10)), allocates a new Buffer object of the specified size. Prior to Node.js 8.0.0, the memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be subsequently initialized by using either buf.fill(0) or by writing to the Buffer completely. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized Buffer versus creating a slower-but-safer Buffer. Starting in Node.js 8.0.0, Buffer(num) and new Buffer(num) will return a Buffer with initialized memory.
  • Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
  • Passing an ArrayBuffer or a SharedArrayBuffer returns a Buffer that shares allocated memory with the given array buffer.

Because the behavior of new Buffer() changes significantly based on the type of value passed as the first argument, applications that do not properly validate the input arguments passed to new Buffer(), or that fail to appropriately initialize newly allocated Buffer content, can inadvertently introduce security and reliability issues into their code.

To make the creation of Buffer instances more reliable and less error prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

Buffer instances returned by Buffer.allocUnsafe() may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize. Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool.

The --zero-fill-buffers command line option#

Added in: v5.10.0

Node.js can be started using the --zero-fill-buffers command line option to force all newly allocated Buffer instances created using either new Buffer(size), Buffer.allocUnsafe(), Buffer.allocUnsafeSlow() or new SlowBuffer(size) to be automatically zero-filled upon creation. Use of this flag changes the default behavior of these methods and can have a significant impact on performance. Use of the --zero-fill-buffers option is recommended only when necessary to enforce that newly allocated Buffer instances cannot contain potentially sensitive data.

Example:

$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>

What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?#

When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow(), the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read.

While there are clear performance advantages to using Buffer.allocUnsafe(), extra care must be taken in order to avoid introducing security vulnerabilities into an application.

Buffers and Character Encodings#

History
VersionChanges
v6.4.0

Introduced latin1 as an alias for binary.

v5.0.0

Removed the deprecated raw and raws encodings.

Buffer instances are commonly used to represent sequences of encoded characters such as UTF-8, UCS2, Base64, or even Hex-encoded data. It is possible to convert back and forth between Buffer instances and ordinary JavaScript strings by using an explicit character encoding.

Example:

const buf = Buffer.from('hello world', 'ascii');

// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('hex'));

// Prints: aGVsbG8gd29ybGQ=
console.log(buf.toString('base64'));

The character encodings currently supported by Node.js include:

  • 'ascii' - For 7-bit ASCII data only. This encoding is fast and will strip the high bit if set.

  • 'utf8' - Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8.

  • 'utf16le' - 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported.

  • 'ucs2' - Alias of 'utf16le'.

  • 'base64' - Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC4648, Section 5.

  • 'latin1' - A way of encoding the Buffer into a one-byte encoded string (as defined by the IANA in RFC1345, page 63, to be the Latin-1 supplement block and C0/C1 control codes).

  • 'binary' - Alias for 'latin1'.

  • 'hex' - Encode each byte as two hexadecimal characters.

Note: Today's browsers follow the WHATWG Encoding Standard which aliases both 'latin1' and 'ISO-8859-1' to 'win-1252'. This means that while doing something like http.get(), if the returned charset is one of those listed in the WHATWG specification it is possible that the server actually returned 'win-1252'-encoded data, and using 'latin1' encoding may incorrectly decode the characters.

Buffers and TypedArray#

History
VersionChanges
v3.0.0

The Buffers class now inherits from Uint8Array.

Buffer instances are also Uint8Array instances. However, there are subtle incompatibilities with TypedArray. For example, while ArrayBuffer#slice() creates a copy of the slice, the implementation of Buffer#slice() creates a view over the existing Buffer without copying, making Buffer#slice() far more efficient.

It is also possible to create new TypedArray instances from a Buffer with the following caveats:

  1. The Buffer object's memory is copied to the TypedArray, not shared.

  2. The Buffer object's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4], not a Uint32Array with a single element [0x1020304] or [0x4030201].

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypeArray object's .buffer property.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`
const buf1 = Buffer.from(arr);

// Shares memory with `arr`
const buf2 = Buffer.from(arr.buffer);

// Prints: <Buffer 88 a0>
console.log(buf1);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf2);

arr[1] = 6000;

// Prints: <Buffer 88 a0>
console.log(buf1);

// Prints: <Buffer 88 13 70 17>
console.log(buf2);

Note that when creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

Example:

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

// Prints: 16
console.log(buf.length);

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

  • TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function:

Buffers and iteration#

Buffer instances can be iterated over using for..of syntax:

Example:

const buf = Buffer.from([1, 2, 3]);

// Prints:
//   1
//   2
//   3
for (const b of buf) {
  console.log(b);
}

Additionally, the buf.values(), buf.keys(), and buf.entries() methods can be used to create iterators.

Class: Buffer#

The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

new Buffer(array)#

History
VersionChanges
v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(array) instead.

Allocates a new Buffer using an array of octets.

Example:

// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'
const buf = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

new Buffer(arrayBuffer[, byteOffset[, length]])#

History
VersionChanges
v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

The byteOffset and length parameters are supported now.

v6.0.0

Deprecated since: v6.0.0

v3.0.0

Added in: v3.0.0

Stability: 0 - Deprecated: Use Buffer.from(arrayBuffer[, byteOffset [, length]]) instead.

This creates a view of the ArrayBuffer or SharedArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`
const buf = new Buffer(arr.buffer);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf);

// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;

// Prints: <Buffer 88 13 70 17>
console.log(buf);

new Buffer(buffer)#

History
VersionChanges
v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(buffer) instead.

Copies the passed buffer data onto a new Buffer instance.

Example:

const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1);

buf1[0] = 0x61;

// Prints: auffer
console.log(buf1.toString());

// Prints: buffer
console.log(buf2.toString());

new Buffer(size)#

History
VersionChanges
v8.0.0

new Buffer(size) will return zero-filled memory by default.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.alloc() instead (also see Buffer.allocUnsafe()).
  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If the size is larger than buffer.constants.MAX_LENGTH or smaller than 0, a