ArrayBuffer.transfer

The static ArrayBuffer.transfer() method returns a new ArrayBuffer whose contents have been taken from the oldBuffer's data and then is either truncated or zero-extended by newByteLength. If newByteLength is undefined, the byteLength of the oldBuffer is used. This operation leaves oldBuffer in a detached state.

Syntax

ArrayBuffer.transfer(oldBuffer [, newByteLength]);

Parameters

oldBuffer

An ArrayBuffer object from which to transfer.

newByteLength

The byte length of the new ArrayBuffer object.

Return value

A new ArrayBuffer object.

Description

The ArrayBuffer.transfer() method allows you to grow and detach ArrayBuffer objects. The ability to grow an ArrayBuffer without copying has the advantage of being much faster for large buffers (similar to realloc). The ability to detach an ArrayBuffer gives the developer explicit control over when the underlying memory is released. This avoids having to drop all references and wait for garbage collection.

Examples

var buf1 = new ArrayBuffer(40);
new Int32Array(buf1)[0] = 42;

var buf2 = ArrayBuffer.transfer(buf1, 80);
buf1.byteLength; // 0 but if you use the polyfill then the value is still 40
buf2.byteLength; // 80
new Int32Array(buf2)[0]; // 42

var buf3 = ArrayBuffer.transfer(buf2, 0);
buf2.byteLength; // 0 but if you use the polyfill then the value is still 80
buf3.byteLength; // 0

Polyfill

You can partially work around this by inserting the following code at the beginning of your scripts, allowing use of much of the functionality of transfer() in implementations that do not natively support it. This is not the exact equivalent of this API, but this function transfers data from one ArrayBuffer to another ArrayBuffer.

if (!ArrayBuffer.transfer) {
    ArrayBuffer.transfer = function(source, length) {
        source = Object(source);
        var dest = new ArrayBuffer(length);
        if (!(source instanceof ArrayBuffer) || !(dest instanceof ArrayBuffer)) {
            throw new TypeError('Source and destination must be ArrayBuffer instances');
        }
        if (dest.byteLength >= source.byteLength) {
            var nextOffset = 0;
            var leftBytes = source.byteLength;
            var wordSizes = [8, 4, 2, 1];
            wordSizes.forEach(function(_wordSize_) {
                if (leftBytes >= _wordSize_) {
                    var done = transferWith(_wordSize_, source, dest, nextOffset, leftBytes);
                    nextOffset = done.nextOffset;
                    leftBytes = done.leftBytes;
                }
            });
        }
        return dest;
        function transferWith(wordSize, source, dest, nextOffset, leftBytes) {
            var ViewClass = Uint8Array;
            switch (wordSize) {
                case 8:
                    ViewClass = Float64Array;
                    break;
                case 4:
                    ViewClass = Float32Array;
                    break;
                case 2:
                    ViewClass = Uint16Array;
                    break;
                case 1:
                    ViewClass = Uint8Array;
                    break;
                default:
                    ViewClass = Uint8Array;
                    break;
            }
            var view_source = new ViewClass(source, nextOffset, Math.trunc(leftBytes / wordSize));
            var view_dest = new ViewClass(dest, nextOffset, Math.trunc(leftBytes / wordSize));
            for (var i = 0; i < view_dest.length; i++) {
                view_dest[i] = view_source[i];
            }
            return {
                nextOffset : view_source.byteOffset + view_source.byteLength,
                leftBytes : source.byteLength - (view_source.byteOffset + view_source.byteLength)
            }
        }
    };
}

Specifications

Not part of any current specification draft document, but has been proposed for a future ECMA-262 edition.

Browser compatibility

See also

• JavaScript typed arrays