getCurrentLetter.js

function getCurrentBlock(editorState) {
    const currentSelection = editorState.getSelection();
    const blockKey = currentSelection.getStartKey();
    return(editorState.getCurrentContent().getBlockForKey(blockKey));
}

function getCurrentLetter(editorState) {
    const currentBlock = getCurrentBlock(editorState);
    const blockText = currentBlock.getText();
    return blockText[editorState.getSelection().getStartOffset() - 1];
}