[["$r","MaxWidth","10",{"children":[["$r","Intro",null,{"children":["$r","p",null,{"children":"State variables might look like regular JavaScript variables that you can read and write to. However, state behaves more like a snapshot. Setting it does not change the state variable you already have, but instead triggers a re-render."}]}],"\n",["$r","YouWillLearn",null,{"children":["$r","ul",null,{"children":["\n",["$r","li",null,{"children":"How setting state triggers re-renders"}],"\n",["$r","li",null,{"children":"When and how state updates"}],"\n",["$r","li",null,{"children":"Why state does not update immediately after you set it"}],"\n",["$r","li",null,{"children":"How event handlers access a “snapshot” of the state"}],"\n"]}]}],"\n",["$r","h2",null,{"id":"setting-state-triggers-renders","children":"Setting state triggers renders "}],"\n",["$r","p",null,{"children":["You might think of your user interface as changing directly in response to the user event like a click. In React, it works a little differently from this mental model. On the previous page, you saw that ",["$r","a",null,{"href":"/learn/render-and-commit#step-1-trigger-a-render","children":"setting state requests a re-render"}]," from React. This means that for an interface to react to the event, you need to ",["$r","em",null,{"children":"update the state"}],"."]}],"\n",["$r","p",null,{"children":["In this example, when you press “send”, ",["$r","code",null,{"children":"setIsSent(true)"}]," tells React to re-render the UI:"]}],"\n"]}],["$r","Sandpack",null,{"children":[["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"import { useState } from 'react';\n\nexport default function Form() {\n const [isSent, setIsSent] = useState(false);\n const [message, setMessage] = useState('Hi!');\n if (isSent) {\n return

Your message is on its way!

\n }\n return (\n

{\n e.preventDefault();\n setIsSent(true);\n sendMessage(message);\n }}>\n setMessage(e.target.value)}\n />\n \n \n );\n}\n\nfunction sendMessage(message) {\n // ...\n}\n"}]}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-css","children":"label, textarea { margin-bottom: 10px; display: block; }\n"}]}]]}],["$r","MaxWidth","28",{"children":["\n",["$r","p",null,{"children":"Here’s what happens when you click the button:"}],"\n",["$r","ol",null,{"children":["\n",["$r","li",null,{"children":["The ",["$r","code",null,{"children":"onSubmit"}]," event handler executes."]}],"\n",["$r","li",null,{"children":[["$r","code",null,{"children":"setIsSent(true)"}]," sets ",["$r","code",null,{"children":"isSent"}]," to ",["$r","code",null,{"children":"true"}]," and queues a new render."]}],"\n",["$r","li",null,{"children":["React re-renders the component according to the new ",["$r","code",null,{"children":"isSent"}]," value."]}],"\n"]}],"\n",["$r","p",null,{"children":"Let’s take a closer look at the relationship between state and rendering."}],"\n",["$r","h2",null,{"id":"rendering-takes-a-snapshot-in-time","children":"Rendering takes a snapshot in time "}],"\n",["$r","p",null,{"children":[["$r","a",null,{"href":"/learn/render-and-commit#step-2-react-renders-your-components","children":"“Rendering”"}]," means that React is calling your component, which is a function. The JSX you return from that function is like a snapshot of the UI in time. Its props, event handlers, and local variables were all calculated ",["$r","strong",null,{"children":"using its state at the time of the render."}]]}],"\n",["$r","p",null,{"children":"Unlike a photograph or a movie frame, the UI “snapshot” you return is interactive. It includes logic like event handlers that specify what happens in response to inputs. React updates the screen to match this snapshot and connects the event handlers. As a result, pressing a button will trigger the click handler from your JSX."}],"\n",["$r","p",null,{"children":"When React re-renders a component:"}],"\n",["$r","ol",null,{"children":["\n",["$r","li",null,{"children":"React calls your function again."}],"\n",["$r","li",null,{"children":"Your function returns a new JSX snapshot."}],"\n",["$r","li",null,{"children":"React then updates the screen to match the snapshot your function returned."}],"\n"]}],"\n"]}],["$r","IllustrationBlock",null,{"sequential":true,"children":[["$r","Illustration",null,{"caption":"React executing the function","src":"/images/docs/illustrations/i_render1.png"}],["$r","Illustration",null,{"caption":"Calculating the snapshot","src":"/images/docs/illustrations/i_render2.png"}],["$r","Illustration",null,{"caption":"Updating the DOM tree","src":"/images/docs/illustrations/i_render3.png"}]]}],["$r","MaxWidth","32",{"children":["\n",["$r","p",null,{"children":["As a component’s memory, state is not like a regular variable that disappears after your function returns. State actually “lives” in React itself—as if on a shelf!—outside of your function. When React calls your component, it gives you a snapshot of the state for that particular render. Your component returns a snapshot of the UI with a fresh set of props and event handlers in its JSX, all calculated ",["$r","strong",null,{"children":"using the state values from that render!"}]]}],"\n"]}],["$r","IllustrationBlock",null,{"sequential":true,"children":[["$r","Illustration",null,{"caption":"You tell React to update the state","src":"/images/docs/illustrations/i_state-snapshot1.png"}],["$r","Illustration",null,{"caption":"React updates the state value","src":"/images/docs/illustrations/i_state-snapshot2.png"}],["$r","Illustration",null,{"caption":"React passes a snapshot of the state value into the component","src":"/images/docs/illustrations/i_state-snapshot3.png"}]]}],["$r","MaxWidth","38",{"children":["\n",["$r","p",null,{"children":["Here’s a little experiment to show you how this works. In this example, you might expect that clicking the “+3” button would increment the counter three times because it calls ",["$r","code",null,{"children":"setNumber(number + 1)"}]," three times."]}],"\n",["$r","p",null,{"children":"See what happens when you click the “+3” button:"}],"\n"]}],["$r","Sandpack",null,{"children":[["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"import { useState } from 'react';\n\nexport default function Counter() {\n const [number, setNumber] = useState(0);\n\n return (\n <>\n

{number}

\n \n \n )\n}\n"}]}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-css","children":"button { display: inline-block; margin: 10px; font-size: 20px; }\nh1 { display: inline-block; margin: 10px; width: 30px; text-align: center; }\n"}]}]]}],["$r","MaxWidth","66",{"children":["\n",["$r","p",null,{"children":["Notice that ",["$r","code",null,{"children":"number"}]," only increments once per click!"]}],"\n",["$r","p",null,{"children":[["$r","strong",null,{"children":["Setting state only changes it for the ",["$r","em",null,{"children":"next"}]," render."]}]," During the first render, ",["$r","code",null,{"children":"number"}]," was ",["$r","code",null,{"children":"0"}],". This is why, in ",["$r","em",null,{"children":"that render’s"}]," ",["$r","code",null,{"children":"onClick"}]," handler, the value of ",["$r","code",null,{"children":"number"}]," is still ",["$r","code",null,{"children":"0"}]," even after ",["$r","code",null,{"children":"setNumber(number + 1)"}]," was called:"]}],"\n",["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"\n"}]}],"\n",["$r","p",null,{"children":"Here is what this button’s click handler tells React to do:"}],"\n",["$r","ol",null,{"children":["\n",["$r","li",null,{"children":[["$r","code",null,{"children":"setNumber(number + 1)"}],": ",["$r","code",null,{"children":"number"}]," is ",["$r","code",null,{"children":"0"}]," so ",["$r","code",null,{"children":"setNumber(0 + 1)"}],".","\n",["$r","ul",null,{"children":["\n",["$r","li",null,{"children":["React prepares to change ",["$r","code",null,{"children":"number"}]," to ",["$r","code",null,{"children":"1"}]," on the next render."]}],"\n"]}],"\n"]}],"\n",["$r","li",null,{"children":[["$r","code",null,{"children":"setNumber(number + 1)"}],": ",["$r","code",null,{"children":"number"}]," is ",["$r","code",null,{"children":"0"}]," so ",["$r","code",null,{"children":"setNumber(0 + 1)"}],".","\n",["$r","ul",null,{"children":["\n",["$r","li",null,{"children":["React prepares to change ",["$r","code",null,{"children":"number"}]," to ",["$r","code",null,{"children":"1"}]," on the next render."]}],"\n"]}],"\n"]}],"\n",["$r","li",null,{"children":[["$r","code",null,{"children":"setNumber(number + 1)"}],": ",["$r","code",null,{"children":"number"}]," is ",["$r","code",null,{"children":"0"}]," so ",["$r","code",null,{"children":"setNumber(0 + 1)"}],".","\n",["$r","ul",null,{"children":["\n",["$r","li",null,{"children":["React prepares to change ",["$r","code",null,{"children":"number"}]," to ",["$r","code",null,{"children":"1"}]," on the next render."]}],"\n"]}],"\n"]}],"\n"]}],"\n",["$r","p",null,{"children":["Even though you called ",["$r","code",null,{"children":"setNumber(number + 1)"}]," three times, in ",["$r","em",null,{"children":"this render’s"}]," event handler ",["$r","code",null,{"children":"number"}]," is always ",["$r","code",null,{"children":"0"}],", so you set the state to ",["$r","code",null,{"children":"1"}]," three times. This is why, after your event handler finishes, React re-renders the component with ",["$r","code",null,{"children":"number"}]," equal to ",["$r","code",null,{"children":"1"}]," rather than ",["$r","code",null,{"children":"3"}],"."]}],"\n",["$r","p",null,{"children":["You can also visualize this by mentally substituting state variables with their values in your code. Since the ",["$r","code",null,{"children":"number"}]," state variable is ",["$r","code",null,{"children":"0"}]," for ",["$r","em",null,{"children":"this render"}],", its event handler looks like this:"]}],"\n",["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"\n"}]}],"\n",["$r","p",null,{"children":["For the next render, ",["$r","code",null,{"children":"number"}]," is ",["$r","code",null,{"children":"1"}],", so ",["$r","em",null,{"children":"that render’s"}]," click handler looks like this:"]}],"\n",["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"\n"}]}],"\n",["$r","p",null,{"children":["This is why clicking the button again will set the counter to ",["$r","code",null,{"children":"2"}],", then to ",["$r","code",null,{"children":"3"}]," on the next click, and so on."]}],"\n",["$r","h2",null,{"id":"state-over-time","children":"State over time "}],"\n",["$r","p",null,{"children":"Well, that was fun. Try to guess what clicking this button will alert:"}],"\n"]}],["$r","Sandpack",null,{"children":[["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"import { useState } from 'react';\n\nexport default function Counter() {\n const [number, setNumber] = useState(0);\n\n return (\n <>\n

{number}

\n \n To:{' '}\n setTo(e.target.value)}>\n \n \n \n \n setMessage(e.target.value)}\n />\n \n \n );\n}\n"}]}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-css","children":"label, textarea { margin-bottom: 10px; display: block; }\n"}]}]]}],["$r","MaxWidth","98",{"children":["\n",["$r","p",null,{"children":[["$r","strong",null,{"children":"React keeps the state values “fixed” within one render’s event handlers."}]," You don’t need to worry whether the state has changed while the code is running."]}],"\n",["$r","p",null,{"children":["But what if you wanted to read the latest state before a re-render? You’ll want to use a ",["$r","a",null,{"href":"/learn/queueing-a-series-of-state-updates","children":"state updater function"}],", covered on the next page!"]}],"\n",["$r","Recap",null,{"children":["$r","ul",null,{"children":["\n",["$r","li",null,{"children":"Setting state requests a new render."}],"\n",["$r","li",null,{"children":"React stores state outside of your component, as if on a shelf."}],"\n",["$r","li",null,{"children":["When you call ",["$r","code",null,{"children":"useState"}],", React gives you a snapshot of the state ",["$r","em",null,{"children":"for that render"}],"."]}],"\n",["$r","li",null,{"children":"Variables and event handlers don’t “survive” re-renders. Every render has its own event handlers."}],"\n",["$r","li",null,{"children":["Every render (and functions inside it) will always “see” the snapshot of the state that React gave to ",["$r","em",null,{"children":"that"}]," render."]}],"\n",["$r","li",null,{"children":"You can mentally substitute state in event handlers, similarly to how you think about the rendered JSX."}],"\n",["$r","li",null,{"children":"Event handlers created in the past have the state values from the render in which they were created."}],"\n"]}]}],"\n"]}],["$r","Challenges",null,{"children":[["$r","h4",null,{"id":"implement-a-traffic-light","children":"Implement a traffic light "}],["$r","p",null,{"children":"Here is a crosswalk light component that toggles when the button is pressed:"}],["$r","Sandpack",null,{"children":[["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"import { useState } from 'react';\n\nexport default function TrafficLight() {\n const [walk, setWalk] = useState(true);\n\n function handleClick() {\n setWalk(!walk);\n }\n\n return (\n <>\n \n

\n {walk ? 'Walk' : 'Stop'}\n

\n \n );\n}\n"}]}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-css","children":"h1 { margin-top: 20px; }\n"}]}]]}],["$r","p",null,{"children":["Add an ",["$r","code",null,{"children":"alert"}]," to the click handler. When the light is green and says “Walk”, clicking the button should say “Stop is next”. When the light is red and says “Stop”, clicking the button should say “Walk is next”."]}],["$r","p",null,{"children":["Does it make a difference whether you put the ",["$r","code",null,{"children":"alert"}]," before or after the ",["$r","code",null,{"children":"setWalk"}]," call?"]}],["$r","Solution",null,{"children":[["$r","p",null,{"children":["Your ",["$r","code",null,{"children":"alert"}]," should look like this:"]}],["$r","Sandpack",null,{"children":[["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"import { useState } from 'react';\n\nexport default function TrafficLight() {\n const [walk, setWalk] = useState(true);\n\n function handleClick() {\n setWalk(!walk);\n alert(walk ? 'Stop is next' : 'Walk is next');\n }\n\n return (\n <>\n \n

\n {walk ? 'Walk' : 'Stop'}\n

\n \n );\n}\n"}]}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-css","children":"h1 { margin-top: 20px; }\n"}]}]]}],["$r","p",null,{"children":["Whether you put it before or after the ",["$r","code",null,{"children":"setWalk"}]," call makes no difference. That render’s value of ",["$r","code",null,{"children":"walk"}]," is fixed. Calling ",["$r","code",null,{"children":"setWalk"}]," will only change it for the ",["$r","em",null,{"children":"next"}]," render, but will not affect the event handler from the previous render."]}],["$r","p",null,{"children":"This line might seem counter-intuitive at first:"}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"alert(walk ? 'Stop is next' : 'Walk is next');\n"}]}],["$r","p",null,{"children":["But it makes sense if you read it as: “If the traffic light shows ‘Walk now’, the message should say ‘Stop is next.‘” The ",["$r","code",null,{"children":"walk"}]," variable inside your event handler matches that render’s value of ",["$r","code",null,{"children":"walk"}]," and does not change."]}],["$r","p",null,{"children":["You can verify that this is correct by applying the substitution method. When ",["$r","code",null,{"children":"walk"}]," is ",["$r","code",null,{"children":"true"}],", you get:"]}],["$r","pre",null,{"children":["$r","code",null,{"className":"language-js","children":"\n

\n Walk\n

\n"}]}],["$r","p",null,{"children":["So clicking “Change to Stop” queues a render with ",["$r","code",null,{"children":"walk"}]," set to ",["$r","code",null,{"children":"false"}],", and alerts “Stop is next”."]}]]}]]}]]