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for loop

Loops can execute code blocks several times. JavaScript knows different kinds of loops. The simplest one is the for loop.
var sum = 0;
for (var i = 1; i < 3; i++) {
  sum = sum + i;
}
The for loop consists of three control expressions and the loop code. The three control expressions follow the for keyword in parentheses. They are separated by semicolons. The first expression is the start expression. It is executed once at the beginning of the loop. The so-called loop variable is normally initialized here. The second expression is the loop condition. It is evaluated to true or false before each loop iteration. If this expression evaluates to true, the loop code is executed. If this expression evaluates to false, the loop is terminated and the program execution continues after the loop. The third expression, called the final expression, is executed after each loop iteration. The loop variable is normally increased here. Then a new loop iteration is started with a new evaluation of the loop condition. The loop code follows the three control expressions in brackets. In our example sum has the value 0 initially. In the start expression i is initialized with 1. The loop condition 1 < 3 is fulfilled, so the loop code is executed. sum receives the value 0 + 1, i. e. 1. Now in the final expression i is increased by 1. (i++ is identical to i = i + 1). i thus receives the value 2. The second loop starts with the evaluation of the loop condition. This condition (2 < 3) is still fulfilled. The loop code is executed again and sum is now given the value 1 + 2, that is 3. In the final expression i is increased again and gets the value 3. The loop condition 3 < 3 is no longer fulfilled. The loop is terminated and the program execution continues after the loop. Our code example has added all natural numbers smaller than 3.

Exercise

Write a function addTo that accepts a number as a parameter and adds all natural numbers smaller or equal than the parameter. The result is to be returned. addTo(3) should return 1+2+3 = 6.
var addTo = function(n) {
  var sum = 0;
  for (var i = 0; i <= n; i++) {
    ...
  }
  return sum;
};
var addTo = function(n) {
  var sum = 0;
  for (var i = 0; i <= n; i++) {
    sum = sum + i;
  }
  return sum;
};