SPA or Single Page Application is a web application implemented in a way it gives the end-user an experience similar to using an installed app. Thus page transitions happen swiftly without the page reloading.
SPAs start by loading up the initial page made up of HTML, CSS, Javascript then rely on Javascript [↗] to pull in data needed to populate subsequent pages. The markup that make up the other pages can be pulled in during the initial page load or constructed using Javascript.
Form submissions are also handled by Javascript. Frameworks [→] such as VueJS [↗], ReactJS [↗], AngularJS [↗] provide alot of what is needed to create SPA out of the box.
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