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JavaScript Typed Arrays: Beginners Guide

by Mozilla ContributorsDecember 18th, 2020

Too Long; Didn't Read

JavaScript typed arrays are array-like objects that provide a mechanism for reading and writing raw binary data in memory buffers. 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.

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JavaScript typed arrays are array-like objects that provide a mechanism for reading and writing raw binary data in memory buffers. 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. This is where typed arrays come in. Each entry in a JavaScript typed array is a raw binary value in one of a number of supported formats, from 8-bit integers to 64-bit floating-point numbers.

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 the
number of elements — that turns the data into a 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.

Int8Array

Description: 8-bit two's complement signed integer
Value Range:
```
-128
```
to
```
127
```
Size in bytes: 1
Web IDL type:
```
byte
```
Equivalent C type:
```
int8_t
```

Uint8Array

Description: 8-bit unsigned integer
Value Range:
```
0
```
to
```
255
```
Size in bytes: 1
Web IDL type:
```
octet
```
Equivalent C type:
```
uint8_t
```

Uint8ClampedArray

Description: 8-bit unsigned integer (clamped)
Value Range:
```
0
```
to
```
255
```
Size in bytes: 1
Web IDL type:
```
octet
```
Equivalent C type:
```
uint8_t
```

Int16Array

Description: 16-bit two's complement signed integer
Value Range:
```
-32768
```
to
```
32767
```
Size in bytes: 2
Web IDL type:
```
short
```
Equivalent C type:
```
int16_t
```

Uint16Array

Description: 16-bit unsigned integer
Value Range:
```
0
```
to
```
65535
```
Size in bytes: 2
Web IDL type:
```
unsigned short
```
Equivalent C type:
```
uint16_t
```

Int32Array

Description: 32-bit two's complement signed integer
Value Range:
```
-2147483648
```
to
```
2147483647
```
Size in bytes: 4
Web IDL type:
```
long
```
Equivalent C type:
```
int32_t
```

Uint32Array

Description: 32-bit unsigned integer
Value Range:
```
0
```
to
```
4294967295
```
Size in bytes: 4
Web IDL type:
```
unsigned long
```
Equivalent C type:
```
uint32_t
```

Float32Array

Description: 32-bit IEEE floating point number (7 significant digits e.g.,
```
1.123456
```
)
Value Range:
```
1.2×10^-38
```
to
```
3.4×10^38
```
Size in bytes: 4
Web IDL type:
```
unrestricted float
```
Equivalent C type:
```
float
```

Float64Array

Description: 64-bit IEEE floating point number (16 significant digits e.g.,
```
1.123...15
```
)
Value Range:
```
5.0×10^-324
```
to
```
1.8×10^308
```
Size in bytes: 8
Web IDL type:
```
unrestricted double
```
Equivalent C type:
```
double
```

BigInt64Array

Description: 64-bit two's complement signed integer
Value Range:
```
-2^63
```
to
```
2^63-1
```
Size in bytes: 8
Web IDL type:
```
bigint
```
Equivalent C type:
```
int64_t (signed long long)
```

BigUint64Array

Description: 64-bit unsigned integer
Value Range:
```
0
```
to
```
2^64-1
```
Size in bytes: 8
Web IDL type:
```
bigint
```
Equivalent C type:
```
uint64_t (unsigned long long)
```

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

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

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

and

inclusive.

Examples

Using views with buffers

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

let 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:

let 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

, and

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:

let 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

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 viewed 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:

let buffer = new ArrayBuffer(24);

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

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

Then you can access, for example, the amount due with

amountDueView[0]

Note: The data structure alignment in a C structure is platform-dependent. Take precautions and considerations for these padding differences.

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 be done using

Array.from()

, or using the following code where

Array.from()

is unsupported.

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