https://techdevguide.withgoogle.com/paths/foundational/stringsplosion-problem-ccocodcode/#!

stringSplosion.js

"use strict";

const assert = require("assert");

(() => {
  const inouts = {
    Code: "CCoCodCode",
    abc: "aababc",
    ab: "aab"
  };

  for (const input in inouts) {
    if (inouts.hasOwnProperty(input)) {
      const output = inouts[input];
      assert.equal(output, stringSplosion(input), "fail");
    }
  }
})();

/**
 * Given a non-empty string like "Code" return a string like "CCoCodCode".

stringSplosion("Code") → "CCoCodCode"
stringSplosion("abc") → "aababc"
stringSplosion("ab") → "aab"
 */
function stringSplosion(str) {
  let strx = "";

  const l = str.length;
  for (let i = 1; i <= l; i++) {
    strx += str.substr(0, i);
  }

  return strx;
}