A Little Bit about For-loops

a loop is a programming language features that repeats a block of code some number of times

for example, this is a JavaScript for-loop that prints Hello 5 times on the console:

for(let i = 0; i < 5; ++i) {
    console.log('Hello')
}

loops can be represented visually as flow charts, e.g.

here’s another for-loop that prints the numbers from 1 to 10 on the screen:

for(let i = 1; i <= 10; ++i) {
    console.log(i)
}

recall that ++i increments i by 1; you can replace that with some other increment, e.g.:

for(let i = 1; i <= 10; i += 2) {  // i += 2 increments i by 2
    console.log(i)
}

the above for-loop prints this:

1
3
4
7
9

this for-loop prints the numbers from 10 down to 1:

for(let i = 10; i >= 1; --i) {
    console.log(i)
}

in general, this kind of for-loop has this format:

for(let i = init_val; stopping_test; increment) {
    // ... loop body ...
}

the variable i is the traditional variable used in a for-loop, and is short for “index”; you could use any other (sensible!) variable name instead of i if you prefer

the stopping_test is any boolean expression that evaluates to either true or false; it usually somehow involves i (as in the examples above)

the increment is how i will be updated at the end of each execution of the loop body; this is usually a statement like ++i, i+=2, --i, etc.

if you want to repeat a block of code, say, 100 times, then this for-loop is a standard way to do that:

for(let i = 0; i < 100; ++i) {   // repeats the loop body 100 times
    // ... loop body ...
}

it’s important to notice that i is initialized to 0 (not 1), and the stopping test is i < 100 (not i <= 100)

starting at 0 is quite common in programming, especially in for-loops, and it works well with other parts of JavaScript