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javascript/snippets/replace-or-append.md

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+---
+title: Replace or append array value
+type: snippet
+tags: [array]
+author: chalarangelo
+cover: boutique-home-office-2
+dateModified: 2023-02-19T05:00:00-04:00
+---
+
+Replaces an item in an array or appends it, if it doesn't exist.
+
+- Use the spread operator (`...`) to create a shallow copy of the array.
+- Use `Array.prototype.findIndex()` to find the index of the first element that satisfies the provided comparison function, `compFn`.
+- If no such element is found, use `Array.prototype.push()` to append the new value to the array.
+- Otherwise, use `Array.prototype.splice()` to replace the value at the found index with the new value.
+
+```js
+const replaceOrAppend = (arr, val, compFn) => {
+  const res = [...arr];
+  const i = arr.findIndex(v => compFn(v, val));
+  if (i === -1) res.push(val);
+  else res.splice(i, 1, val);
+  return res;
+};
+```
+
+```js
+const people = [ { name: 'John', age: 30 }, { name: 'Jane', age: 28 } ];
+const jane = { name: 'Jane', age: 29 };
+const jack = { name: 'Jack', age: 28 };
+replaceOrAppend(people, jane, (a, b) => a.name === b.name);
+// [ { name: 'John', age: 30 }, { name: 'Jane', age: 29 } ]
+replaceOrAppend(people, jack, (a, b) => a.name === b.name);
+// [
+//   { name: 'John', age: 30 },
+//   { name: 'Jane', age: 28 },
+//   { name: 'Jack', age: 28 }
+// ]
+```