Get width and height of item with fixed aspect ratio fitted in arbitrary container

fit-with-aspect-ratio.js

// @flow
type Size = { width: number, height: number };

function getFittedSlideSize(container: Size, target: Size): Size {
    const targetAspectRatio = target.width / target.height;
    const containerAspectRatio = container.width / container.height;
    // if aspect ratio of target is "wider" then target's aspect ratio
    const fit = targetAspectRatio > containerAspectRatio
        ? 'width' // fit by width, so target's width = container's width
        : 'height'; // fit by height, so target's height = container's height

    return {
        width: fit === 'width'
            ? containerWidth
            : Math.round(containerHeight * ( target.width / target.height)),
        height: fit === 'height'
            ? containerHeight
            : Math.round(containerWidth * (target.height / target.width)),
    };
}