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JavaScript typed arrays are array-like objects and provide a mechanism for accessing raw binary data. As you may already know, Array
objects grow and shrink dynamically and can have any JavaScript value. JavaScript engines perform optimizations so that these arrays are fast. However, as web applications become more and more powerful, adding features such as audio and video manipulation, access to raw data using WebSockets, and so forth, it has become clear that there are times when it would be helpful for JavaScript code to be able to quickly and easily manipulate raw binary data in typed arrays.
However, typed arrays are not to be confused with normal arrays, as calling Array.isArray()
on a typed array returns false
. Moreover, not all methods available for normal arrays are supported by typed arrays (e.g. push and pop).
Buffers and views: typed array architecture
To achieve maximum flexibility and efficiency, JavaScript typed arrays split the implementation into buffers and views. A buffer (implemented by the ArrayBuffer
object) is an object representing a chunk of data; it has no format to speak of, and offers no mechanism for accessing its contents. In order to access the memory contained in a buffer, you need to use a view. A view provides a context — that is, a data type, starting offset, and number of elements — that turns the data into an actual typed array.
ArrayBuffer
The ArrayBuffer
is a data type that is used to represent a generic, fixed-length binary data buffer. You can't directly manipulate the contents of an ArrayBuffer
; instead, you create a typed array view or a DataView
which represents the buffer in a specific format, and use that to read and write the contents of the buffer.
Typed array views
Typed array views have self descriptive names and provide views for all the usual numeric types like Int8
, Uint32
, Float64
and so forth. There is one special typed array view, the Uint8ClampedArray
. It clamps the values between 0 and 255. This is useful for Canvas data processing, for example.
Type | Size in bytes | Description | Web IDL type | Equivalent C type |
Int8Array |
1 | 8-bit two's complement signed integer | byte |
int8_t |
Uint8Array |
1 | 8-bit unsigned integer | octet |
uint8_t |
Uint8ClampedArray |
1 | 8-bit unsigned integer (clamped) | octet |
uint8_t |
Int16Array |
2 | 16-bit two's complement signed integer | short |
int16_t |
Uint16Array |
2 | 16-bit unsigned integer | unsigned short |
uint16_t |
Int32Array |
4 | 32-bit two's complement signed integer | long |
int32_t |
Uint32Array |
4 | 32-bit unsigned integer | unsigned long |
uint32_t |
Float32Array |
4 | 32-bit IEEE floating point number | unrestricted float |
float |
Float64Array |
8 | 64-bit IEEE floating point number | unrestricted double |
double |
DataView
The DataView
is a low-level interface that provides a getter/setter API to read and write arbitrary data to the buffer. This is useful when dealing with different types of data, for example. Typed array views are in the native byte-order (see Endianness) of your platform. With a DataView
you are able to control the byte-order. It is big-endian by default and can be set to little-endian in the getter/setter methods.
Web APIs using typed arrays
FileReader.prototype.readAsArrayBuffer()
- The
FileReader.prototype.readAsArrayBuffer()
method starts reading the contents of the specifiedBlob
orFile
. XMLHttpRequest.prototype.send()
XMLHttpRequest
instances'send()
method now supports typed arrays andArrayBuffer
objects as argument.ImageData.data
- Is a
Uint8ClampedArray
representing a one-dimensional array containing the data in the RGBA order, with integer values between0
and255
inclusive.
Examples
Using views with buffers
First of all, we will need to create a buffer, here with a fixed length of 16-bytes:
var buffer = new ArrayBuffer(16);
At this point, we have a chunk of memory whose bytes are all pre-initialized to 0. There's not a lot we can do with it, though. We can confirm that it is indeed 16 bytes long, and that's about it:
if (buffer.byteLength === 16) { console.log("Yes, it's 16 bytes."); } else { console.log("Oh no, it's the wrong size!"); }
Before we can really work with this buffer, we need to create a view. Let's create a view that treats the data in the buffer as an array of 32-bit signed integers:
var int32View = new Int32Array(buffer);
Now we can access the fields in the array just like a normal array:
for (var i = 0; i < int32View.length; i++) { int32View[i] = i * 2; }
This fills out the 4 entries in the array (4 entries at 4 bytes each makes 16 total bytes) with the values 0, 2, 4, and 6.
Multiple views on the same data
Things start to get really interesting when you consider that you can create multiple views onto the same data. For example, given the code above, we can continue like this:
var int16View = new Int16Array(buffer); for (var i = 0; i < int16View.length; i++) { console.log("Entry " + i + ": " + int16View[i]); }
Here we create a 16-bit integer view that shares the same buffer as the existing 32-bit view and we output all the values in the buffer as 16-bit integers. Now we get the output 0, 0, 2, 0, 4, 0, 6, 0.
You can go a step farther, though. Consider this:
int16View[0] = 32; console.log("Entry 0 in the 32-bit array is now " + int32View[0]);
The output from this is "Entry 0 in the 32-bit array is now 32". In other words, the two arrays are indeed simply views on the same data buffer, treating it as different formats. You can do this with any view types.
Working with complex data structures
By combining a single buffer with multiple views of different types, starting at different offsets into the buffer, you can interact with data objects containing multiple data types. This lets you, for example, interact with complex data structures from WebGL, data files, or C structures you need to use while using js-ctypes.
Consider this C structure:
struct someStruct { unsigned long id; char username[16]; float amountDue; };
You can access a buffer containing data in this format like this:
var buffer = new ArrayBuffer(24); // ... read the data into the buffer ... var idView = new Uint32Array(buffer, 0, 1); var usernameView = new Uint8Array(buffer, 4, 16); var amountDueView = new Float32Array(buffer, 20, 1);
Then you can access, for example, the amount due with amountDueView[0]
.
Conversion to normal arrays
After processing a typed array, it is sometimes useful to convert it back to a normal array in order to benefit from the Array
prototype. This can done using Array.from
, or using the following code where Array.from
is unsupported.
var typedArray = new Uint8Array([1, 2, 3, 4]), normalArray = Array.prototype.slice.call(typedArray); normalArray.length === 4; normalArray.constructor === Array;
Specifications
Specification | Status | Comment |
---|---|---|
Typed Array Specification | Obsolete | Superseded by ECMAScript 6. |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'TypedArray Objects' in that specification. |
Standard | Initial definition in an ECMA standard. |
ECMAScript 2017 Draft (ECMA-262) The definition of 'TypedArray Objects' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | 7.0 | 4.0 (2) | 10 | 11.6 | 5.1 |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | 4.0 | (Yes) | 4.0 (2) | 10 | 11.6 | 4.2 |