JavaScript typed arrays

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.

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 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, Uint8ClampedArray, which clamps the values between 0 and 255. This is useful for Canvas data processing, for example.

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.

These are some examples of APIs that make use of typed arrays; there are others, and more are being added all the time.

FileReader.prototype.readAsArrayBuffer()

The FileReader.prototype.readAsArrayBuffer() method starts reading the contents of the specified Blob or File.

XMLHttpRequest.prototype.send()

XMLHttpRequest instances' send() method now supports typed arrays and ArrayBuffer objects as argument.

ImageData.data

Is a Uint8ClampedArray representing a one-dimensional array containing the data in the RGBA order, with integer values between 0 and 255 inclusive.

First of all, we will need to create a buffer, here with a fixed length of 16-bytes:

const 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:

const int32View = new Int32Array(buffer);

Now we can access the fields in the array just like a normal array:

for (let 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.

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:

const int16View = new Int16Array(buffer);

for (let 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 viewed on the same data buffer, treating it as different formats. You can do this with any view types.

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 or data files.

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:

const buffer = new ArrayBuffer(24);

// ... read the data into the buffer ...

const idView = new Uint32Array(buffer, 0, 1);
const usernameView = new Uint8Array(buffer, 4, 16);
const amountDueView = new Float32Array(buffer, 20, 1);

Then you can access, for example, the amount due with amountDueView[0].

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 be done using Array.from()

const typedArray = new Uint8Array([1, 2, 3, 4]);
const normalArray = Array.from(typedArray);

as well as the spread syntax

const typedArray = new Uint8Array([1, 2, 3, 4]);
const normalArray = [...typedArray];

or using the following code where Array.from() is unsupported.

const typedArray = new Uint8Array([1, 2, 3, 4]);
const normalArray = Array.prototype.slice.call(typedArray);

• Getting ArrayBuffers or typed arrays from Base64-encoded strings
• Faster Canvas Pixel Manipulation with Typed Arrays
• Typed Arrays: Binary Data in the Browser
• Endianness

• « Previous
• Next »