JavaScript For Loop-很有趣，可能很简单

Question

I've recently completed a challenge in Marjin Haverbeke's book, " Eloquent Javascript, Second Edition ".

One had to create this console output using a for loop:

#
##
###
####
#####
######
#######

The answer is this:

for (var i = '#'; i.length < 8; i += '#') {
console.log(i);
}

What I would like to know is why the first line is not two hashes ('##') as surely the updating section of the loop (i += '#') adds '#' to i (already = to '#'), therefor meaning that the first iteration of the loop should surely output '##'?

Perhaps I need a lesson on how this loop really works.

Yours truly, still seemingly JS newbie.

Answer 1

for (init(); condition(); update()) {
  body();
}

is equivalent to

init();
while (condition()) {
  body();
  update();
}

Thus, the first time i += '#' runs is only after the first console.log(i) (when i was just '#' ).

Answer 2

@Callum you should first check how for loop works.

if you have written .

for (var i = '#'; i.length < 8; i += '#') {
console.log(i);
}

so what happen in this loop is this .

1 ) first var i ='#' initiallize

then condition

2) i.length < 8

then it goes to executed the statement

3) console.log(i);

4) then increment

i += '#'

then from step 2 to 4

this is what for loops algorithm works

Answer 3

it is just like post increment. The order of execution is:

--> initialisation
--> check condition
--> execute body
--> increment value

so first it would print the value then increment it.

Answer 4

This would give ## in first row:

for (var i = '#'; i += '#',i.length < 8; ) {
console.log(i);
}