What Actually Happens When Your Frontend Code Runs in the Browser
A lot of frontend confusion comes from one simple thing: Most beginners write code without really knowing what the browser is doing with it. So things feel mysterious. Re-renders feel random. Bugs feel unpredictable. Let’s slow it down and walk through what actually happens, without going low-level or adding too much detail. Step 1: The browser builds a map of the page When your HTML loads, the browser doesn’t just “show a page.” It builds a structure called the DOM (Document Object Model). Think of this as the browser’s internal tree of: - elements - text - relationships This is what the browser uses to understand what exists on the page. Step 2: CSS tells the browser how things should look Next, the browser applies CSS. CSS doesn’t change structure, it answers questions like: - Where should this be positioned? - How big is it? - What color, font, and spacing should it use? The browser combines HTML + CSS to calculate layout and paint pixels on the screen. That’s why small CSS changes can cause big visual shifts. Step 3: JavaScript adds behavior JavaScript is where frontend becomes dynamic. When your JavaScript runs, it can: - Read data from the page - Listen for events (clicks, input, scroll) - Change state - Update the DOM Important point: JavaScript doesn’t magically “update the UI.” It changes data, and the browser reacts by updating what’s rendered. That’s the core idea behind how frontend code works. Step 4: Events trigger change When a user clicks a button or types into an input: - An event fires - Your code runs - State changes - The browser updates the UI This loop happens constantly. Frontend development is really about managing what changes, when it changes, and what depends on it. Step 5: Re-rendering isn’t magic In modern frameworks like React, the browser isn’t updating everything. Instead, React is: - Comparing what should be shown - Updating only what changed That’s why understanding browser rendering basics makes React feel less mysterious.