title: Extending object's properties without overwriting them
body: <p>I'm trying to extend the keys/values in target object (with the keys/values) from source object, but without overwriting existing keys/values in the target object. Meaning: </p>
  
  <pre><code>var obj1 = {
    a: 1,
    b: 2
  };
  var obj2 = {
    b: 4,
    c: 3
  };
  extend(obj1, obj2);
  console.log(obj1); // --&gt; {a: 1, b: 2, c: 3}
  </code></pre>
  
  <p>Interestingly, I found <code>Object.assign(obj1,obj2);</code>, but it overwrites the keys/values.</p>
  
  <p>I need to <strong>not overwrite them, if existent</strong> and <strong>add them if nonexistent</strong>.</p>
  
  <p>Please help in plain JavaScript.</p>