Added some js programs

AirthmaticOperators.js

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+let x=5;
+let y=4;
+let sum=x+y;
+let product=x*y;
+let division=x/y;
+let subs=x-y;
+console.log("Sum is =",sum);
+console.log("Substraction is =",subs);
+console.log("Multiplication is =",product);
+console.log("Division is =",division);

HalfPyramid.js

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+function createHalfPyramid (height) {
+
+  for (var i = 1; i <= height; i++) {
+    var row = '';
+
+    for (var j = 1; j <= i; j++) {
+      row += '*';
+    }
+
+    console.log(row);
+  }
+}
+
+createHalfPyramid(5);

HelloWorld.js

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+console.log("Hello, World!");