darts: Add new exercise

exercises/target/README.md

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+# Target
+
+Write a function that returns the earned points in a Darts game.
+
+[Darts](https://en.wikipedia.org/wiki/Darts) is a game where players
+throw darts to a [target](https://en.wikipedia.org/wiki/Darts#/media/File:Darts_in_a_dartboard.jpg).
+
+In our particular instance of the game, the target rewards with 4 different amount of points, depending on where the dart lands:
+
+* If the dart lands outside the target, player earns no points (0 points).
+* If the dart lands in the outer circle of the target, player earns 1 point.
+* If the dart lands in the middle circle of the target, player earns 5 points.
+* If the dart lands in the inner circle of the target, player earns 10 points.
+
+The outer circle has a radius of 10 units (This is equivalent to the total radius for the entire target), the middle circle a radius of 5 units, and the inner circle a radius of 1. Of course, they are all centered to the same point (That is, the circles are [concentrics](http://mathworld.wolfram.com/ConcentricCircles.html)).
+
+Write a function that given a point in the target (defined by its `real` cartesian coordinates `x` and `y`), returns the correct amount earnt by a dart landing in that point.
+
+## Setup
+
+Go through the setup instructions for Javascript to
+install the necessary dependencies:
+
+[https://exercism.io/tracks/javascript/installation](https://exercism.io/tracks/javascript/installation)
+
+## Requirements
+
+Install assignment dependencies:
+
+```bash
+$ npm install
+```
+
+## Making the test suite pass
+
+Execute the tests with:
+
+```bash
+$ npm test
+```
+
+In the test suites all tests but the first have been skipped.
+
+Once you get a test passing, you can enable the next one by
+changing `xtest` to `test`.
+
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/target/example.js

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+export default function solve(x, y) {
+  // Define as numbers
+  x = Number(x);
+  y = Number(y);
+
+  // Check for NaN
+  if (x !== x || y !== y) return null;
+
+  // Use euclidean distance
+  const distanceToDart = Math.sqrt(x*x + y*y);
+
+  // Define points for each section of the target
+  if (distanceToDart > 10) return 0;
+  else if (distanceToDart > 5) return 1;
+  else if (distanceToDart > 1) return 5;
+  return 10;
+}

exercises/target/package.json

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+{
+  "name": "exercism-javascript",
+  "version": "0.0.0",
+  "description": "Exercism exercises in Javascript.",
+  "author": "Katrina Owen",
+  "private": true,
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/exercism/javascript"
+  },
+  "devDependencies": {
+    "babel-eslint": "^10.0.1",
+    "babel-jest": "^21.2.0",
+    "babel-plugin-transform-builtin-extend": "^1.1.2",
+    "babel-preset-env": "^1.4.0",
+    "eslint": "^5.6.0",
+    "eslint-config-airbnb-base": "^13.1.0",
+    "eslint-plugin-import": "^2.14.0",
+    "jest": "^23.6.0"
+  },
+  "jest": {
+    "modulePathIgnorePatterns": [
+      "package.json"
+    ]
+  },
+  "babel": {
+    "presets": [
+      [
+        "env",
+        {
+          "targets": [
+            {
+              "node": "current"
+            }
+          ]
+        }
+      ]
+    ],
+    "plugins": [
+      [
+        "babel-plugin-transform-builtin-extend",
+        {
+          "globals": [
+            "Error"
+          ]
+        }
+      ],
+      [
+        "transform-regenerator"
+      ]
+    ]
+  },
+  "scripts": {
+    "test": "jest --no-cache ./*",
+    "watch": "jest --no-cache --watch ./*",
+    "lint": "eslint .",
+    "lint-test": "eslint . && jest --no-cache ./* "
+  },
+  "eslintConfig": {
+    "parser": "babel-eslint",
+    "parserOptions": {
+      "ecmaVersion": 7,
+      "sourceType": "module"
+    },
+    "env": {
+      "es6": true,
+      "node": true,
+      "jest": true
+    },
+    "extends": "airbnb-base",
+    "rules": {
+      "import/no-unresolved": "off",
+      "import/extensions": "off",
+      "import/prefer-default-export": "off",
+      "import/no-default-export": "off"
+    }
+  },
+  "license": "MIT",
+  "dependencies": {}
+}

exercises/target/target.spec.js

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+import solve from './target';
+
+describe('Solve the target problem', () => {
+  test('A dart lands outside the target', () => {
+    const x = 15.3;
+    const y = 13.2;
+    const expected = 0;
+    expect(solve(x, y)).toEqual(expected);
+  });
+
+  test('A dart lands just in the border of the target', () => {
+    const x = 10;
+    const y = 0;
+    const expected = 1;
+    expect(solve(x, y)).toEqual(expected);
+  });
+
+  test('Input is not a number', () => {
+    const x = 'WRONG';
+    const y = 10;
+    expect(solve(x, y)).toBeNull();
+  });
+
+  test('A dart lands in the middle circle', () => {
+    const x = 3;
+    const y = 3.7;
+    const expected = 5;
+    expect(solve(x, y)).toEqual(expected);
+  });
+
+  test('A dart lands right in the border between outside and middle circles', () => {
+    const x = 0;
+    const y = 5;
+    const expected = 5;
+    expect(solve(x, y)).toEqual(expected);
+  });
+
+  test('A dart arrives in the inner circle', () => {
+    const x = 0;
+    const y = 0;
+    const expected = 10;
+    expect(solve(x, y)).toEqual(expected);
+  });
+});