Implement calculator

calculator.js

@@ -0,0 +1,47 @@
+const operators = ['+', '-', '*', '/'];
+
+exports.create_calculator = function (operator) {
+  const numbers = [];
+
+  return {
+    add_number: function (number) {
+      numbers.push(number);
+    },
+    calculate: function () {
+      // Return zero if there are no numbers to calculate
+      if (numbers.length === 0) {
+        throw 'Cannot calculate a result because no numbers were given';
+      }
+
+      let answer = numbers[0];
+
+      let i = 0;
+      numbers.forEach(function (number) {
+        if (i === 0) {
+          i++;
+          return;
+        }
+
+        switch (operator) {
+          case '+':
+            answer = answer + number;
+            break;
+          case '-':
+            answer = answer - number;
+            break;
+          case '/':
+            answer = answer / number;
+            break;
+          default:
+            console.log("Invalid operator " + operator);
+        }
+        // console.log('index: ' + index);
+        // console.log('current answer: ' + answer);
+
+        i++;
+      });
+
+      return answer;
+    }
+  }
+}

index.js

@@ -1,7 +1,20 @@
 const readline = require('readline-sync');
+const calc = require('./calculator');
 
 console.log('Welcome to the calculator!');
 
-console.log('Please enter some input:');
-const response = readline.prompt();
-console.log('You entered: ' + response);
+console.log('Enter the operator:');
+const operator = readline.prompt();
+
+console.log(`How many numbers do you want to ${operator} ?`);
+const c = parseInt(readline.prompt());
+
+const calculator = calc.create_calculator(operator);
+
+for (var i = 0; i < c; i++) {
+  console.log('Enter a number:');
+  const n = parseInt(readline.prompt());
+  calculator.add_number(n);
+}
+
+console.log('The answer is: ' + calculator.calculate());

test/calculator_test.js

@@ -0,0 +1,44 @@
+const assert = require('assert');
+const chai = require('chai');
+const calc = require('../calculator')
+
+describe('calculator', function () {
+  describe('#calculate()', function () {
+    it('adds one number', function () {
+      const calculator = calc.create_calculator('+');
+
+      calculator.add_number(16);
+
+      assert.equal(16, calculator.calculate());
+    });
+
+    it('adds two numbers', function () {
+      const calculator = calc.create_calculator('+');
+
+      calculator.add_number(5);
+      calculator.add_number(4);
+
+      assert.equal(9, calculator.calculate());
+    });
+
+    it('adds many numbers', function () {
+      const calculator = calc.create_calculator('+');
+
+      calculator.add_number(5);
+      calculator.add_number(4);
+      calculator.add_number(0);
+      calculator.add_number(-20);
+      calculator.add_number(1486);
+      calculator.add_number(-402);
+      calculator.add_number(4);
+
+      assert.equal(1077, calculator.calculate());
+    });
+
+    it('rejects the calculation when there are no numbers', function () {
+      const calculator = calc.create_calculator('+');
+
+      chai.expect(calculator.calculate).to.throw('Cannot calculate a result because no numbers were given');
+    });
+  });
+});