getRequestedFields.js

const graphql = require('graphql');

function getRequestedFields(query, info) {
    const parsed = graphql.parse(query);

    if (parsed.definitions.length > 1) {
        throw new Error('Too many definitions');
    }

    const requested = info.operation;

    let nodeA = parsed.definitions[0];
    let nodeB = requested;

    // eslint-disable-next-line no-constant-condition
    while (true) {
        if (nodeA.selectionSet && nodeA.selectionSet.selections.length > 1) {
            throw new Error('Too many selections');
        }

        if (!nodeA.name) {
            nodeA = nodeA.selectionSet.selections[0];
            continue;
        }

        if (!nodeB.selectionSet) {
            return [];
        }

        if (nodeA.name && nodeA.name.value === '__requestedFields') {
            return nodeB.selectionSet.selections.map(x => x.name.value);
        }

        // eslint-disable-next-line no-loop-func
        nodeB = nodeB.selectionSet.selections.find(x => {
            return x.name.value === nodeA.name.value;
        });

        if (!nodeB) {
            return [];
        }

        if (!nodeA.selectionSet) {
            throw new Error('Missing __requestedFields field');
        }

        nodeA = nodeA.selectionSet.selections[0];
    }
}

module.exports = getRequestedFields;