Slate relative offset

relativeOffset.js

/**
 * Converts a {key, offset} combination to a single offset relative to
 * `node`. `key` must be the key of one of the texts inside `node`.
 */
function relativeOffset(node, key, offset) {
    return node
        .getTexts()
        .takeUntil(text => text.key === key)
        .reduce((absOffset, text) => absOffset + text.length)
        + offset;
}