Node.js solution for week 101, part 2

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diff --git a/challenge-101/abigail/node/ch-2.js b/challenge-101/abigail/node/ch-2.js
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+#!/usr/local/bin/node
+
+//
+// See ../README.md
+//
+
+//
+// Run as: node ch-2.js < input-file
+//
+
+//
+// See https://stackoverflow.com/questions/2049582/
+//
+
+//
+// This determines on which side of the line through (x1, y1) and
+// (x2, y2) the origin lies. If > 0, then the origin lies to the left
+// of the line, if < 0, the origin lies to the right of the line, if
+// = 0, the origin lies on the line.
+//
+function side (x1, y1, x2, y2) {
+    return (y2 - y1) * x2 - (x2 - x1) * y2
+}
+
+
+require ('readline')
+. createInterface ({input: process . stdin})   
+. on ('line', _ => {
+    //
+    // Parse input
+    //
+    let [x1, y1, x2, y2, x3, y3] = _ . split (/\s+/)
+
+    //
+    // Determine where the origin is relative to the three lines 
+    // through the vertices of the triangle. Note we have to go
+    // in a specific order through the points. (Either clock wise,
+    // or counter clockwise, as long as we're consistent).
+    //
+    let s1 = side (x2, y2, x3, y3)
+    let s2 = side (x3, y3, x1, y1)
+    let s3 = side (x1, y1, x2, y2)
+
+    //
+    // If the origin either lies to the left (or on) each of the
+    // lines, or to the right (or on) each of the lines, the origin
+    // lies inside the triangle. If not, it does not.
+    //
+    console . log ((s1 <= 0 && s2 <= 0 && s3 <= 0) ||
+                   (s1 >= 0 && s2 >= 0 && s3 >= 0) ? 1 : 0)
+});