Self-growing code codee codeee codeeee
JavaScript (ES6), 14 12 bytes

-2 bytes thanks to @Shaggy

f=_=&gt;"f=_"+f


Testing snippet



s=`f=_=&gt;"f=_"+f`;

for(i=10;i--;s=eval(s)())
  o.innerText+=s+"\n";
&lt;pre id=o&gt;&lt;/pre&gt;