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.

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

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.

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.

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;
}

DataView()

Creates a new DataView object.

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 the DataView constructor.

DataView.prototype[@@toStringTag]

The initial value of the @@toStringTag property is the string "DataView". This property is used in Object.prototype.toString().

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 floating point number.

DataView.prototype.getFloat64()

Reads 8 bytes starting at the specified byte offset of this DataView and interprets them as a 64-bit floating point number.

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.

• Polyfill of DataView in core-js
• ArrayBuffer
• SharedArrayBuffer