Resizes a base64 image to a specified maximum largest dimension (whether width or height)

resize-image.js

/**
 * Resizes a picture to a maximum length/width (based on largest dimension)
 * @param {string} original Base64 representation of image to be resized.
 * @param {number} maxSize Amount of pixels that largest dimention (whether width or length) should have.
 * @param {string} finalMimeType Mime type of the resulting resized image.
 * @returns {Promise} A Promise that resolves with resized picture.
 */
resizeImage(original, maxSize, finalMimeType = 'image/jpeg') {
	return new Promise(resolve => {
		const image = new Image();
	
		image.onload = () =>
		{
			// Resize the image
			const canvas = document.createElement('canvas');
			let width = image.width,
				height = image.height;
	
			if (width > height && width > maxSize)
			{
				height *= maxSize / width;
				width = maxSize;
			}
			else if (height > maxSize)
			{
				width *= maxSize / height;
				height = maxSize;
			}
			canvas.width = width;
			canvas.height = height;
			canvas.getContext('2d').drawImage(image, 0, 0, width, height);
	
			const base64str = canvas.toDataURL(finalMimeType);
	
			resolve(base64str);
		};
	
		image.src = original;
	});
}