Mixins for Checkout estimation.js file Magento 2.

We can create mixins for Magento_Checkout/js/view/estimation file to modify the core Javascript method or add a new method to the file.

To create mixins, first, create a requirejs-config.js file in the module view/frontend area.

FilePath:
app/code/Jesadiya/Estimation/view/frontend/requirejs-config.js

var config = {
    config: {
        mixins: {
            /**
             * Mixin for always show sidebar.
             */
            'Magento_Checkout/js/view/estimation': {
                'Jesadiya_Estimation/js/view/estimation-mixin': true
            }
    }
};

Create Mixins js file to modify the core module’s original file with our changes,
app/code/Jesadiya/Estimation/view/frontend/web/js/view/estimation-mixin.js

define([
    'jquery',
    'Magento_Checkout/js/model/sidebar'
], function (
    $,
    sidebarModel
) {
    'use strict';

    return function (Estimation) {
        return Estimation.extend({
            /**
             * Show sidebar.
             */
            showSidebar: function () {
                // you can add custom logic before show sidebar
            },
        });
    }
});

You can add your custom logic before show sidebar with mixins,

Using the above way you can also modify other functions of estimation.js in the checkout module.