adding library

.gitignore

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+node_modules/

package.json

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+{
+  "name": "simple-random-string",
+  "version": "1.0.0",
+  "description": "get a random string",
+  "main": "src/index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/yiliashaw/simple-random-string.git"
+  },
+  "keywords": [
+    "random",
+    "string"
+  ],
+  "author": "yiliashaw <yilia.shaw@gmail.com>",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/yiliashaw/simple-random-string/issues"
+  },
+  "homepage": "https://github.com/yiliashaw/simple-random-string#readme",
+  "dependencies": {
+    "unique-random-array": "1.0.1"
+  }
+}

src/index.js

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+var uniqueRandomArray = require('unique-random-array');
+
+module.exports = function (len) {
+  var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+  var optionArray = possible.split('');
+  var length = len && typeof len === 'number' && len > 0 ? len : 8;
+  var str = '';
+  var rand = uniqueRandomArray(optionArray);
+  for (var i = 0; i < length; i++) {
+    str += rand();
+  }
+  return str;
+};