DataView
The DataView
view provides a low-level interface for reading and writing multiple number types in a binary ArrayBuffer
, without having to care about the platform's endianness.
Description
Endianness
Multi-byte number formats are represented in memory differently depending on machine architecture — see Endianness for an explanation. DataView
accessors provide explicit control of how data is accessed, regardless of the executing computer's endianness.
js
const littleEndian = (() => {
const buffer = new ArrayBuffer(2);
new DataView(buffer).setInt16(0, 256, true /* littleEndian */);
// Int16Array uses the platform's endianness.
return new Int16Array(buffer)[0] === 256;
})();
console.log(littleEndian); // true or false
64-bit Integer Values
Some browsers don't have support for DataView.prototype.setBigInt64()
and DataView.prototype.setBigUint64()
. So to enable 64-bit operations in your code that will work across browsers, you could implement your own getUint64()
function, to obtain values with precision up to Number.MAX_SAFE_INTEGER
— which could suffice for certain cases.
js
function getUint64(dataview, byteOffset, littleEndian) {
// split 64-bit number into two 32-bit (4-byte) parts
const left = dataview.getUint32(byteOffset, littleEndian);
const right = dataview.getUint32(byteOffset + 4, littleEndian);
// combine the two 32-bit values
const combined = littleEndian
? left + 2 ** 32 * right
: 2 ** 32 * left + right;
if (!Number.isSafeInteger(combined))
console.warn(combined, "exceeds MAX_SAFE_INTEGER. Precision may be lost");
return combined;
}
Alternatively, if you need full 64-bit range, you can create a BigInt
. Further, although native BigInts are much faster than user-land library equivalents, BigInts will always be much slower than 32-bit integers in JavaScript due to the nature of their variable size.
js
const BigInt = window.BigInt,
bigThirtyTwo = BigInt(32),
bigZero = BigInt(0);
function getUint64BigInt(dataview, byteOffset, littleEndian) {
// split 64-bit number into two 32-bit (4-byte) parts
const left = BigInt(dataview.getUint32(byteOffset | 0, !!littleEndian) >>> 0);
const right = BigInt(
dataview.getUint32(((byteOffset | 0) + 4) | 0, !!littleEndian) >>> 0,
);
// combine the two 32-bit values and return
return littleEndian
? (right << bigThirtyTwo) | left
: (left << bigThirtyTwo) | right;
}
Constructor
DataView()
-
Creates a new
DataView
object.
Instance properties
These properties are defined on DataView.prototype
and shared by all DataView
instances.
DataView.prototype.buffer
-
The
ArrayBuffer
referenced by this view. Fixed at construction time and thus read only. DataView.prototype.byteLength
-
The length (in bytes) of this view. Fixed at construction time and thus read only.
DataView.prototype.byteOffset
-
The offset (in bytes) of this view from the start of its
ArrayBuffer
. Fixed at construction time and thus read only. DataView.prototype.constructor
-
The constructor function that created the instance object. For
DataView
instances, the initial value is theDataView
constructor. DataView.prototype[@@toStringTag]
-
The initial value of the
@@toStringTag
property is the string"DataView"
. This property is used inObject.prototype.toString()
.
Instance methods
DataView.prototype.getBigInt64()
-
Reads 8 bytes starting at the specified byte offset of this
DataView
and interprets them as a 64-bit signed integer. DataView.prototype.getBigUint64()
-
Reads 8 bytes starting at the specified byte offset of this
DataView
and interprets them as a 64-bit unsigned integer. DataView.prototype.getFloat32()
-
Reads 4 bytes starting at the specified byte offset of this
DataView
and interprets them as a 32-bit float. DataView.prototype.getFloat64()
-
Reads 8 bytes starting at the specified byte offset of this
DataView
and interprets them as a 64-bit float. DataView.prototype.getInt16()
-
Reads 2 bytes starting at the specified byte offset of this
DataView
and interprets them as a 16-bit signed integer. DataView.prototype.getInt32()
-
Reads 4 bytes starting at the specified byte offset of this
DataView
and interprets them as a 32-bit signed integer. DataView.prototype.getInt8()
-
Reads 1 byte at the specified byte offset of this
DataView
and interprets it as an 8-bit signed integer. DataView.prototype.getUint16()
-
Reads 2 bytes starting at the specified byte offset of this
DataView
and interprets them as a 16-bit unsigned integer. DataView.prototype.getUint32()
-
Reads 4 bytes starting at the specified byte offset of this
DataView
and interprets them as a 32-bit unsigned integer. DataView.prototype.getUint8()
-
Reads 1 byte at the specified byte offset of this
DataView
and interprets it as an 8-bit unsigned integer. DataView.prototype.setBigInt64()
-
Takes a BigInt and stores it as a 64-bit signed integer in the 8 bytes starting at the specified byte offset of this
DataView
. DataView.prototype.setBigUint64()
-
Takes a BigInt and stores it as a 64-bit unsigned integer in the 8 bytes starting at the specified byte offset of this
DataView
. DataView.prototype.setFloat32()
-
Takes a number and stores it as a 32-bit float in the 4 bytes starting at the specified byte offset of this
DataView
. DataView.prototype.setFloat64()
-
Takes a number and stores it as a 64-bit float in the 8 bytes starting at the specified byte offset of this
DataView
. DataView.prototype.setInt16()
-
Takes a number and stores it as a 16-bit signed integer in the 2 bytes at the specified byte offset of this
DataView
. DataView.prototype.setInt32()
-
Takes a number and stores it as a 32-bit signed integer in the 4 bytes at the specified byte offset of this
DataView
. DataView.prototype.setInt8()
-
Takes a number and stores it as an 8-bit signed integer in the byte at the specified byte offset of this
DataView
. DataView.prototype.setUint16()
-
Takes a number and stores it as a 16-bit unsigned integer in the 2 bytes at the specified byte offset of this
DataView
. DataView.prototype.setUint32()
-
Takes a number and stores it as a 32-bit unsigned integer in the 4 bytes at the specified byte offset of this
DataView
. DataView.prototype.setUint8()
-
Takes a number and stores it as an 8-bit unsigned integer in the byte at the specified byte offset of this
DataView
.
Examples
Using DataView
js
const buffer = new ArrayBuffer(16);
const view = new DataView(buffer, 0);
view.setInt16(1, 42);
view.getInt16(1); // 42
Specifications
Specification |
---|
ECMAScript Language Specification # sec-dataview-objects |
Browser compatibility
BCD tables only load in the browser
See also
- Polyfill of
DataView
incore-js
- jDataView: JavaScript library that polyfills and extends the
DataView
API to all browsers and Node.js. ArrayBuffer
SharedArrayBuffer