marko/scripts/babel-plugin-marko-debug.js

/**
 * Babel plugin for production builds which converts "MARKO_DEBUG"
 * strings to false to be removed by babel-plugin-minify-dead-code-elimination.
 */
module.exports = function babelPluginMarkoDebug() {
    return {
        visitor: {
            IfStatement: path => {
                const node = path.node;
                return replaceMarkoDebug(
                    path,
                    node.test,
                    node.consequent,
                    node.alternate
                );
            },
            ConditionalExpression: path => {
                const node = path.node;
                return replaceMarkoDebug(
                    path,
                    node.test,
                    node.consequent,
                    node.alternate
                );
            },
            LogicalExpression: path => {
                const node = path.node;

                if (node.operator === "&&") {
                    return replaceMarkoDebug(path, node.left, node.right);
                } else {
                    return replaceMarkoDebug(path, node.left, null, node.right);
                }
            }
        }
    };
};

/**
 * Replaces any conditions containing "MARKO_DEBUG" or !"MARKO_DEBUG" and collapses them.
 */
function replaceMarkoDebug(path, test, consequent, alternate) {
    // Reverse replacement when negating the expression.
    if (test.type === "UnaryExpression" && test.operator === "!") {
        const temp = consequent;
        consequent = alternate;
        alternate = temp;
        test = test.argument;
    }

    // Only look for "MARKO_DEBUG" strings.
    if (test.type !== "StringLiteral") {
        return;
    }

    // If we found a condition that is a string (and isn't "MARKO_DEBUG") it's most likely a mistake.
    if (test.value !== "MARKO_DEBUG") {
        throw new Error(
            `Found if statement with StringLiteral "${test.value}", did you mean "MARKO_DEBUG".`
        );
    }

    if (alternate) {
        if (alternate.type === "BlockStatement") {
            path.replaceWithMultiple(alternate.body);
        } else {
            path.replaceWith(alternate);
        }
    } else {
        path.remove();
    }
}