for

Use the for statement to execute a code block for every entity in selector or entity context. It additionally has a classic for loop syntax.
It’s shorthand for execute command.

Usage

Normal for:

for (@a) {
    >>say I'm a player
}

context players = @a;
for (players) {
    >>say I'm a player
}

for with custom parameters:

for (@p "0 5 0") {
    >>say I'm the closest player to coords 0 5 0
}

This kind of for is compiled into execute @p 0 5 0. This means, that in the same way, you can use other execute command parameters like detect.

Classic for:

for (let i = 0; i < 10; i++) {
    debug("i = " + i);
}