Don't Know how to make calculator history

Question

So I am building a calculator with history but don't really know where to start. I have a full working calculator but I want to change it so that it shows the history of operations a person does. I want the history to remain there for all different operations the user makes. Plus, per example if a user does 1+2+3 I want the full operation to appear: 1+2+3. If the user does a second operation like 2/8+2 I want that the previous operations shows up too: 1+2+3
2/8+2

I already have a space for the history in my html file. Here's the link for the fiddle: https://jsfiddle.net/q9pa0tns/

This is my javascript code so far:

(function() {
  "use strict";

  // Shortcut to get elements
  var el = function(element) {
    if (element.charAt(0) === "#") { // If passed an ID...
      return document.querySelector(element); // ... returns single element
    }

    return document.querySelectorAll(element); // Otherwise, returns a nodelist
  };

  // Variables
  var viewer = el("#viewer"), // Calculator screen where result is displayed
    equals = el("#equals"), // Equal button
    nums = el(".num"), // List of numbers
    ops = el(".ops"), // List of operators
    theNum = "", // Current number
    oldNum = "", // First number
    resultNum, // Result
    operator; // Batman

  // When: Number is clicked. Get the current number selected
  var setNum = function() {
    if (resultNum) { // If a result was displayed, reset number
      theNum = this.getAttribute("data-num");
      resultNum = "";
    } else { // Otherwise, add digit to previous number (this is a string!)
      theNum += this.getAttribute("data-num");
    }

    viewer.innerHTML = theNum; // Display current number

  };

  // When: Operator is clicked. Pass number to oldNum and save operator
  var moveNum = function() {
    oldNum = theNum;
    theNum = "";
    operator = this.getAttribute("data-ops");

    equals.setAttribute("data-result", ""); // Reset result in attr
  };

  // When: Equals is clicked. Calculate result
  var displayNum = function() {

    // Convert string input to numbers
    oldNum = parseFloat(oldNum);
    theNum = parseFloat(theNum);

    // Perform operation
    switch (operator) {
      case "plus":
        resultNum = oldNum + theNum;
        break;

      case "minus":
        resultNum = oldNum - theNum;
        break;

      case "times":
        resultNum = oldNum * theNum;
        break;

      case "divided by":
        resultNum = oldNum / theNum;
        break;

        // If equal is pressed without an operator, keep number and continue
      default:
        resultNum = theNum;
    }

    // If NaN or Infinity returned
    if (!isFinite(resultNum)) {
      if (isNaN(resultNum)) { // If result is not a number; set off by, eg, double-clicking operators
        resultNum = "You broke it!";
      } else { // If result is infinity, set off by dividing by zero
        resultNum = "Look at what you've done";
        el('#calculator').classList.add("broken"); // Break calculator
        el('#reset').classList.add("show"); // And show reset button
      }
    }

    // Display result, finally!
    viewer.innerHTML = resultNum;
    equals.setAttribute("data-result", resultNum);

    // Now reset oldNum & keep result
    oldNum = 0;
    theNum = resultNum;

  };

  // When: Clear button is pressed. Clear everything
  var clearAll = function() {
    oldNum = "";
    theNum = "";
    viewer.innerHTML = "0";
    equals.setAttribute("data-result", resultNum);
  };

  /* The click events */

  // Add click event to numbers
  for (var i = 0, l = nums.length; i < l; i++) {
    nums[i].onclick = setNum;
  }

  // Add click event to operators
  for (var i = 0, l = ops.length; i < l; i++) {
    ops[i].onclick = moveNum;
  }

  // Add click event to equal sign
  equals.onclick = displayNum;

  // Add click event to clear button
  el("#clear").onclick = clearAll;

  // Add click event to reset button
  el("#reset").onclick = function() {
    window.location = window.location;
  };

}());

Any suggestions would be highly appreciated, thank you so much.

Answer 1

For starters

https://jsfiddle.net/mplungjan/b2cagn8h/

....
history = el("#history"), // history
....

var historyString = oldNum + " " + operator + " " + theNum + " = " + resultNum;
if (historyString.indexOf("NaN") ===-1 && historyString.indexOf("undefined") === -1) {
  history.innerHTML += historyString + "<br/>";
}

Then you can look into sessionStorage