elm-hot/src/inject.js

Hot code swapping for Elm

const fs = require('fs');
const path = require('path');

// inject the HMR code into the Elm compiler's JS output
function inject(originalElmCodeJS) {

    const hmrCode = fs.readFileSync(path.join(__dirname, "../resources/hmr.js"), {encoding: "utf8"});

    // first, verify that we have not been given Elm 0.18 code
    if (originalElmCodeJS.indexOf("_elm_lang$core$Native_Platform.initialize") >= 0) {
        throw new Error("[elm-hot] Elm 0.18 is not supported. Please use fluxxu/elm-hot-loader@0.5.x instead.");
    }

    let modifiedCode = originalElmCodeJS;

    if (originalElmCodeJS.indexOf("elm$browser$Browser$application") !== -1) {
        // attach a tag to Browser.Navigation.Key values. It's not really fair to call this a hack
        // as this entire project is a hack, but this is evil evil evil. We need to be able to find
        // the Browser.Navigation.Key in a user's model so that we do not swap out the new one for
        // the old. But as currently implemented (2018-08-19), there's no good way to detect it.
        // So we will add a property to the key immediately after it's created so that we can find it.
        const navKeyDefinition = /var\s+key\s*=\s*function\s*\(\)\s*{\s*key.a\(\s*onUrlChange\(\s*_Browser_getUrl\(\)\s*\)\s*\);\s*};/;
        function replacementString(match) {
            return match + "\n" + "key['elm-hot-nav-key'] = true;";
        }
        modifiedCode = originalElmCodeJS.replace(navKeyDefinition, replacementString);
        if (modifiedCode === originalElmCodeJS) {
            throw new Error("[elm-hot] Browser.Navigation.Key def not found. Version mismatch?");
        }
    }

    // splice in the HMR code
    const regex = /(_Platform_export\([^]*)(}\(this\)\);)/;
    const match = regex.exec(modifiedCode);

    if (match === null) {
        throw new Error("Compiled JS from the Elm compiler is not valid. You must use the Elm 0.19 compiler.");
    }

    return modifiedCode.slice(0, match.index)
        + match[1] + "\n\n" + hmrCode + "\n\n" + match[2];
}

module.exports = {
    inject: inject
};