[AST + JSShiftCode] - "function name" to "window.name = function"

function-to-window.js

function isOnTheBody(path) {
    return path.scope.parent.isGlobal;
}

function transform(j, path) {
    const propName = path.value.id.name;
    const params = path.value.params;
    const body = path.value.body;

    j(path).replaceWith(
        j.expressionStatement(
            j.assignmentExpression(
                "=",
                j.memberExpression(
                    j.identifier("window"),
                    j.identifier(propName)
                ),
                j.functionExpression(
                    j.identifier(propName),
                    params,
                    body
                )
            )
        )
    );
}

module.exports = function(fileInfo, api, options) {
    const j = api.jscodeshift;
    const ast = j(fileInfo.source);

    ast
        .find(j.FunctionDeclaration)
        .filter(isOnTheBody)
        .forEach((path, index) => {
            transform(j, path);
        });

    return ast.toSource();
};

If you have old libraries with functions on the top of the file you will have problems with bundling them by Webpack. This script transforms functions to variables on the window.
Before:
function a() {}
After:
window.a = function a() {};