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+### indexOfAll
+
+Returns all indices of `val` in an array. If `val` never occurs, returns `[-1]`.
+
+Use `Array.forEach()` to loop over elements and `Array.push()` to store indices for matching elements.
+Return `[-1]` if `length` of the array of indices is `0`, otherwise return the array of indices.
+
+``` js
+const indexOfAll = (arr, val) => {
+  const indices = [];
+  arr.forEach((el, i) => el === val && indices.push(i))
+  return indices.length ? indices : [-1];
+};
+
+```
+
+``` js
+indexOfAll([1,2,3,1,2,3],1); // [0,3]
+indexOfAll([1,2,3],4); // [-1]
+```