SharePoint Framework (SPFx)

According to microsoft-

The SharePoint Framework (SPFx) is a page and web part model that provides full support for client-side SharePoint development, easy integration with SharePoint data, and extending Microsoft Teams. With the SharePoint Framework, you can use modern web technologies and tools in your preferred development environment to build productive experiences and apps that are responsive and mobile-ready.

Key features of the SharePoint Framework include:

• It runs in the context of the current user and connection in the browser. There are no iFrames for the customization (JavaScript is embedded directly to the page).
• The controls are rendered in the normal page DOM.
• The controls are responsive and accessible by nature.
• It enables the developer to access the lifecycle in addition to render, load, serialize and de-serialize, configuration changes, and more.
• It's framework-agnostic. You can use any JavaScript framework that you like including, but not limited to, React, Handlebars, Knockout, Angular, and Vue.js.
• The developer toolchain is based on popular open-source client development tools such as NPM, TypeScript, Yeoman, webpack, and gulp.
• Performance is reliable.
• End users can use SPFx client-side solutions that are approved by the tenant administrators (or their delegates) on all sites, including self-service team, group, or personal sites.
• SPFx web parts can be added to both classic and modern pages.
• SPFx solutions can be used to extend Microsoft Teams.