Checking for convex polygon in JavaScript

Convex Polygon

A convex polygon is defined as a polygon with all its interior angles less than 180°.

Problem

We are required to write a JavaScript function that takes in an array of coordinates, basically the array will be an array of arrays each subarray containing exactly two numbers, specifying a point on a 2-D plane.

Our function should determine whether the polygon formed by these points is a convex polygon or not. If yes, the function should return true, false otherwise.

For example, if the input to the function is −

const arr = [[0,0],[0,1],[1,1],[1,0]];

Then the output should be −

const output = true;

Output Explanation:

These points will draw out a perfect square in which all the vertices have an interior angle of 90.

Example

The code for this will be −

const arr = [[0,0],[0,1],[1,1],[1,0]];
const isConvex = (arr = []) => {
   const { length } = arr;
   let pre = 0, curr = 0;
   for (let i = 0; i < length; ++i) {
      let dx1 = arr[(i + 1) % length][0] - arr[i][0];
      let dx2 = arr[(i + 2) % length][0] - arr[(i + 1) % length][0];
      let dy1 = arr[(i + 1) % length][1] - arr[i][1];
      let dy2 = arr[(i + 2) % length][1] - arr[(i + 1) % length][1];
      curr = dx1 * dy2 - dx2 * dy1;
      if (curr != 0) {
         if ((curr > 0 && pre < 0) || (curr < 0 && pre > 0))
            return false;
         else
            pre = curr;
      };
   };
   return true;
};
console.log(isConvex(arr));

Output

And the output in the console will be −

true