JavaScript 类型数组

JavaScript 类型数组 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).

缓冲和视图:类型化数组体系结构

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.

Typed arrays in an ArrayBuffer

ArrayBuffer

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.

类型化数组视图

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 值范围 Size in bytes 描述 Web IDL 类型 Equivalent C type
Int8Array -128 to 127 1 8-bit two's complement signed integer byte int8_t
Uint8Array 0 to 255 1 8-bit unsigned integer octet uint8_t
Uint8ClampedArray 0 to 255 1 8-bit unsigned integer (clamped) octet uint8_t
Int16Array -32768 to 32767 2 16-bit two's complement signed integer short int16_t
Uint16Array 0 to 65535 2 16-bit unsigned integer unsigned short uint16_t
Int32Array -2147483648 to 2147483647 4 32-bit two's complement signed integer long int32_t
Uint32Array 0 to 4294967295 4 32-bit unsigned integer unsigned long uint32_t
Float32Array 1.2 × 10 -38 to 3.4 × 10 38 4 32-bit IEEE floating point number (7 significant digits e.g., 1.123456 ) unrestricted float float
Float64Array 5.0 × 10 -324 to 1.8 × 10 308 8 64-bit IEEE floating point number (16 significant digits e.g., 1.123...15 ) unrestricted double double
BigInt64Array -2 63 to 2 63 -1 8 64-bit two's complement signed integer bigint int64_t (signed long long)
BigUint64Array 0 to 2 64 -1 8 64-bit unsigned integer bigint uint64_t (unsigned long long)

DataView

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.

使用类型化数组的 Web API

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()
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
Uint8ClampedArray representing a one-dimensional array containing the data in the RGBA order, with integer values between 0 and 255 包含在内。

范例

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 0 , 2 , 4 ,和 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:

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 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 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] .

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

转换到正常数组

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;
					

规范

规范
ECMAScript (ECMA-262)
The definition of 'TypedArray Objects' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
Int8Array Chrome 7 Edge 12 Firefox 4 IE 10 Opera 11.6 Safari 5.1 WebView Android 4 Chrome Android 18 Firefox Android 4 Opera Android 12 Safari iOS 4.2 Samsung Internet Android 1.0 nodejs 0.10
Int8Array() 构造函数 Chrome 7 Edge 12 Firefox 4 IE 10 Opera 11.6 Safari 5.1 WebView Android 4 Chrome Android 18 Firefox Android 4 Opera Android 12 Safari iOS 4.2 Samsung Internet Android 1.0 nodejs 0.10

图例

完整支持
完整支持

另请参阅

Metadata

  1. JavaScript
  2. 教程:
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  5. 中间体
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  6. 高级
    1. 继承和原型链
    2. 严格模式
    3. JavaScript 类型数组
    4. 内存管理
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  7. 参考:
  8. 内置对象
    1. AggregateError
    2. Array
    3. ArrayBuffer
    4. AsyncFunction
    5. Atomics
    6. BigInt
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    9. Boolean
    10. DataView
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    15. Float32Array
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    48. Uint8Array
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    48. TypeError: "x" is not a function
    49. TypeError: "x" is not a non-null object
    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
    54. TypeError: X.prototype.y called on incompatible type
    55. TypeError: can't access dead object
    56. TypeError: can't access property "x" of "y"
    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
    60. TypeError: can't redefine non-configurable property "x"
    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
    63. TypeError: invalid 'instanceof' operand 'x'
    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
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    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
  13. 杂项
    1. JavaScript technologies overview
    2. 词汇语法
    3. JavaScript 数据结构
    4. Enumerability and ownership of properties
    5. Iteration protocols
    6. 严格模式
    7. Transitioning to strict mode
    8. Template literals
    9. 弃用特征