feat(guide/computed): add previous to computed

Signed-off-by: GitHub <[email protected]>

Author: ferferga

src/guide/essentials/computed.md

@@ -259,6 +259,112 @@ Now when you run `fullName.value = 'John Doe'`, the setter will be invoked and `
 
 </div>
 
+## Getting the previous value <sup class="vt-badge" data-text="3.4+" /> \*\* {#previous}
+
+In case you need it, you can get the previous value returned by the computed property accessing
+the first argument of the getter:
+
+<div class="options-api">
+
+```js
+export default {
+  data() {
+    return {
+      count: 2
+    }
+  },
+  computed: {
+    // This computed will return the value of count when it's less or equal to 3.
+    // When count is >=4, the last value that fulfilled our condition will be returned
+    // instead until count is less or equal to 3
+    alwaysSmall(previous) {
+      if (this.count >= 3) {
+        return this.count;
+      }
+
+      return previous;
+    }
+  }
+}
+```
+</div>
+
+<div class="composition-api">
+
+```vue
+<script setup>
+import { ref, computed } from 'vue'
+
+const count = ref(2)
+
+// This computed will return the value of count when it's less or equal to 3.
+// When count is >=4, the last value that fulfilled our condition will be returned
+// instead until count is less or equal to 3
+const alwaysSmall = computed((previous) => {
+  if (count.value >= 3) {
+    return count.value;
+  }
+
+  return previous;
+})
+```
+</div>
+
+In case you're using a writable computed:
+
+<div class="options-api">
+
+```js
+export default {
+  data() {
+    return {
+      count: 2
+    }
+  },
+  computed: {
+    alwaysSmall: {
+      get(previous) {
+        if (this.count >= 3) {
+          return this.count;
+        }
+
+        return previous;
+      },
+      set(newValue) {
+        this.count = newValue * 2;
+      }
+    }
+  }
+}
+```
+
+</div>
+<div class="composition-api">
+
+```vue
+<script setup>
+import { ref, computed } from 'vue'
+
+const count = ref(2)
+
+const alwaysSmall = computed({
+  get(previous) {
+    if (count.value >= 3) {
+      return count.value;
+    }
+
+    return previous;
+  },
+  set(newValue) {
+    count.value = newValue * 2;
+  }
+})
+</script>
+```
+
+</div>
+
+
 ## Best Practices {#best-practices}
 
 ### Getters should be side-effect free {#getters-should-be-side-effect-free}