Rotate image preview to compensate for EXIF orientation (Javascript / Typescript)

imageOrientation.ts

// Based on: https://stackoverflow.com/a/46814952/283851
// Based on: https://gist.github.com/mindplay-dk/72f47c1a570e870a375bd3dbcb9328fb
/**
 * Create a Base64 Image URL, with rotation applied to compensate for EXIF orientation, if needed.
 *
 * Optionally resize to a smaller maximum width - to improve performance for larger image thumbnails.
 */
export function getImageUrl(file: File, maxWidth: number|undefined): Promise<string> {
    return readOrientation(file).then(orientation => applyRotation(file, orientation || 1, maxWidth || 999999));
}

/**
 * @returns EXIF orientation value (or undefined)
 */
const readOrientation = (file: File) => new Promise<number|undefined>(resolve => {
    const reader = new FileReader();

    reader.onload = () => resolve((() => {
        const view = new DataView(/** @type {ArrayBuffer} */ (reader.result) as ArrayBuffer);

        if (view.getUint16(0, false) != 0xFFD8) {
            return;
        }

        const length = view.byteLength;

        let offset = 2;

        while (offset < length) {
            const marker = view.getUint16(offset, false);

            offset += 2;

            if (marker == 0xFFE1) {
                offset += 2;

                if (view.getUint32(offset, false) != 0x45786966) {
                    return;
                }

                offset += 6;

                const little = view.getUint16(offset, false) == 0x4949;

                offset += view.getUint32(offset + 4, little);

                const tags = view.getUint16(offset, little);

                offset += 2;

                for (let i = 0; i < tags; i++) {
                    if (view.getUint16(offset + (i * 12), little) == 0x0112) {
                        return view.getUint16(offset + (i * 12) + 8, little);
                    }
                }
            } else if ((marker & 0xFF00) != 0xFF00) {
                break;
            } else {
                offset += view.getUint16(offset, false);
            }
        }
    })());

    reader.readAsArrayBuffer(file.slice(0, 64 * 1024));
});

/**
 * @returns Base64 Image URL (with rotation applied to compensate for orientation, if any)
 */
const applyRotation = (file: File, orientation: number, maxWidth: number) => new Promise<string>(resolve => {
    const reader = new FileReader();

    reader.onload = () => {
        const url = reader.result as string;

        const image = new Image();

        image.onload = () => {
            const canvas = document.createElement("canvas");
            const context = canvas.getContext("2d")!;

            let { width, height } = image;

            const [outputWidth, outputHeight] = orientation >= 5 && orientation <= 8
                ? [height, width]
                : [width, height];

            const scale = outputWidth > maxWidth ? maxWidth / outputWidth : 1;

            width = Math.floor(width * scale);
            height = Math.floor(height * scale);

            // to rotate rectangular image, we need enough space so square canvas is used
            const wh = Math.max(width, height);

            // set proper canvas dimensions before transform & export
            canvas.width = wh;
            canvas.height = wh;

            // for some transformations output image will be aligned to the right or bottom of square canvas
            let rightAligned = false;
            let bottomAligned = false;
            // transform context before drawing image
            switch (orientation) {
                case 2: context.transform(-1, 0, 0, 1, wh, 0); rightAligned = true; break;
                case 3: context.transform(-1, 0, 0, -1, wh, wh); rightAligned = true; bottomAligned = true; break;
                case 4: context.transform(1, 0, 0, -1, 0, wh); bottomAligned = true; break;
                case 5: context.transform(0, 1, 1, 0, 0, 0); break;
                case 6: context.transform(0, 1, -1, 0, wh, 0); rightAligned = true; break;
                case 7: context.transform(0, -1, -1, 0, wh, wh); rightAligned = true; bottomAligned = true; break;
                case 8: context.transform(0, -1, 1, 0, 0, wh); bottomAligned = true; break;
                default: break;
            }

            // draw image
            context.drawImage(image, 0, 0, width, height);

            // copy rotated image to output dimensions and export it
            const canvas2 = document.createElement("canvas");
            canvas2.width = Math.floor(outputWidth * scale);
            canvas2.height = Math.floor(outputHeight * scale);
            const ctx2 = canvas2.getContext("2d");
            const sx = rightAligned ? canvas.width - canvas2.width : 0;
            const sy = bottomAligned ? canvas.height - canvas2.height : 0;
            ctx2.drawImage(canvas, sx, sy, canvas2.width, canvas2.height, 0, 0, canvas2.width, canvas2.height);

            // export base64
            resolve(canvas2.toDataURL("image/jpeg"));
        };

        image.src = url;
    };

    reader.readAsDataURL(file);
});