buildTriangle.js

Build a triangle with loops

function buildTriangle(numLines) {
  var linesArray = [];
  var iterations = numLines;
  
  if (numLines < 1 ) {
    return alert("Try a bigger number!")
  }
  
  for (var line = 1; line <= iterations; line++) {    //start @ 1 instead of zero to avoid empty first index string
  linesArray.push("#".repeat(line) + "\n");
  }
  return console.log(linesArray.join(""));
}

LGTM. I would consider maybe handling numLines with any value less then 1.
Maybe something like..

   // drop in after "var" declarations and before "for" loop
   // if our numLines param is less then 1, then return an empty string
   if (numLines < 1) { 
     return ""; 
   }

Edited. Thanks for the suggestion!

Try this - I think it makes it more readable: https://jsfiddle.net/q9xywy8m/5/