find surrounding block ranges

findBlockRange.js

function areSameBlock(a, b) {
	a.getType() === b.getType()
}

function findBlockRange(contentState, areEqualFn, startBlock) {
	const after = block => contentState.getBlockAfter(block.getKey())
	const before = block => contentState.getBlockBefore(block.getKey())

	const find = (block, nextBlock, directionFn) => {
		if (!nextBlock || !areEqualFn(block, nextBlock)) {
			return block
		}
		return find(nextBlock, directionFn(nextBlock), directionFn)
	}
	return [
		find(startBlock, before(startBlock), before), 
		find(startBlock, after(startBlock), after)
	]
}

// Usage
findBlockRange(contentState, areSameBlock, getCurrentBlock(editorState))