Mixins for Checkout estimation.js file Magento 2.

Magento checkout estimation mixins for the class Magento_Checkout/js/view/estimation 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 a 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 showing the sidebar with mixins,

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

You can check some other mixins as well from the given link.

• • Store pickup mixins
  • Payment list template Override using mixins.js in Magento 2.