@netlify/redirect-parser/lib/merge.js

Parses netlify redirects into a js object representation

import stringify from 'fast-safe-stringify';
import { splitResults } from './results.js';
// Merge redirects from `_redirects` with the ones from `netlify.toml`.
// When both are specified, both are used and `_redirects` has priority.
// Since in both `netlify.toml` and `_redirects`, only the first matching rule
// is used, it is possible to merge `_redirects` to `netlify.toml` by prepending
// its rules to `netlify.toml` `redirects` field.
export const mergeRedirects = function ({ fileRedirects, configRedirects }) {
    const results = [...fileRedirects, ...configRedirects];
    const { redirects, errors } = splitResults(results);
    const mergedRedirects = removeDuplicates(redirects);
    return { redirects: mergedRedirects, errors };
};
// Remove duplicates. This is especially likely considering `fileRedirects`
// might have been previously merged to `configRedirects`, which happens when
// `netlifyConfig.redirects` is modified by plugins.
// The latest duplicate value is the one kept, hence why we need to iterate the
// array backwards and reverse it at the end
const removeDuplicates = function (redirects) {
    const uniqueRedirects = new Set();
    const result = [];
    for (let i = redirects.length - 1; i >= 0; i--) {
        const r = redirects[i];
        const key = stringify.default.stableStringify(r);
        if (uniqueRedirects.has(key))
            continue;
        uniqueRedirects.add(key);
        result.push(r);
    }
    return result.reverse();
};