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javascript/snippets/vector-angle.md

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+---
+title: Vector angle
+type: snippet
+tags: [math]
+cover: avocado-slices
+dateModified: 2021-01-08T00:23:44+02:00
+---
+
+Calculates the angle (theta) between two vectors.
+
+- Use `Array.prototype.reduce()`, `Math.pow()` and `Math.sqrt()` to calculate the magnitude of each vector and the scalar product of the two vectors.
+- Use `Math.acos()` to calculate the arccosine and get the theta value.
+
+```js
+const vectorAngle = (x, y) => {
+  let mX = Math.sqrt(x.reduce((acc, n) => acc + Math.pow(n, 2), 0));
+  let mY = Math.sqrt(y.reduce((acc, n) => acc + Math.pow(n, 2), 0));
+  return Math.acos(x.reduce((acc, n, i) => acc + n * y[i], 0) / (mX * mY));
+};
+```
+
+```js
+vectorAngle([3, 4], [4, 3]); // 0.283794109208328
+```
+