myConcat.html

<script src="..\my-jsArrayMethods\simpletest.js"></script>
<script>
  // Standalone version of the native Array.prototype.concat() method.
  // myConcat is used to merge two or more arrays. 
  // myConcat does not change the existing arrays. 
  // myConcat returns a new array.
  // myConcat copies string, number and bool values into the new array.
  // myConcat copies object references to the new array.
    // if either referenced object is modified, it will affect both the new and original arrays.

  function myConcat(arr1, arr2) {
    const args = Array.from(arguments);
     returnArr = [];

    if (!Array.isArray(arguments[0])) {
      throw new TypeError('The first parameter passed to myConcat must be an array.');
    }
    returnArr[0] = arguments[0];
    for (var i = 1; i < args.length; i++) {
      var el = args[i];
      if (Array.isArray(el)) {
        for (var j = 0; j < el.length; j++){
          returnArr[returnArr.length] = el[j];
        }
      }
    }
    return returnArr;
  }

  tests({
    'It should throw type error if first arg is not an array.': function() {
      var isTypeError = false;
      try {
        var myObj = {name: 'Alex'};
        myConcat(myObj, [1]);
      } 
      catch(e) {
        isTypeError = (e instanceof TypeError);
      }
      eq(isTypeError, true);
    },
    'It should not modify the array arguments.': function() {
      var arr1 = ['a'];
      var arr2 = ['b'];
      var concatArr = myConcat(arr1, arr2);
      eq(arr1.length, 1);
      eq(arr2.length, 1);
      eq(concatArr.length, 2);
    },
    'It should copy all string/number/bool elements from array arguments into new array.': function() {
      var arr1 = ['a'];
      var arr2 = [1];
      var arr3 = [true, false]
      var concatArr = myConcat(arr1, arr2, arr3);
      eq(concatArr[0], 'a');
      eq(concatArr[1], 1);
      eq(concatArr[2], true);
      eq(concatArr[3], false);
    },    
    'It should copy references of nested objects in the array arguments.': function() {
      var arr1 = [1];
      var obj1 = {name: 'Alex'};
      var arrNested = [arr1, obj1];
      var arr2 = [2];
      var concatArr = myConcat(arr2, arrNested);
      eq(concatArr[0], 2);
      eq(concatArr[1], arr1);
      eq(concatArr[2], obj1);
    },    
    'Changes to argument objects should be visible on both new returned array and the original object.': function() {
      var arr1 = [1];
      var obj1 = {name: 'Alex'};
      var arrNested = [arr1, obj1];
      var arr2 = [2];
      var concatArr = myConcat(arr2, arrNested);
      eq(concatArr[2], obj1);
      obj1.name = 'not Alex';
      arr1[1] = 'not 1';
      eq(concatArr[2].name, 'not Alex');
      eq(concatArr[1].length, 2);
    },    
  })
</script>