将for循环条件分配给变量

Question

Is there any way to assign a variable as the conditional operator in a for loop in Javascript?

For example, take this for loop:

for(var i = 0, j = 5; i < 10; i++, j++) {
    console.log(i + ", " + j);
}

I want to be able to set the i < 10 part as a variable. I might want the for loop to run until j < 8 instead of i < 10 . The way I am currently doing it is using an if statement and then using two different for loops with the exact same code except for the conditional, and I just wanted to know if a more efficient way was possible. Thanks!

Answer 1

Use a function in the for loop and reassign it to whatever you want.

var predicate = function(i, j) { return i < 10; }

for(var i = 0, j = 5; predicate(i, j); i++, j++) {
    console.log(i + ", " + j);
    if (i === 5) { // Dummy condition
        predicate = function(i, j) { return j < 8; };
    }
}

The advantage is that you can completely change the logic if you need to.

Answer 2

You can replace the 10 with a variable. You cannot replace the operator ( < ) with a variable.

You can replace the entire expression with a function though, and that function can be stored in a variable.

var data = ["a", "b", "c"];

function loop(callback) {
    for (var i = 0; callback(i); i++) {
        console.log(data[i]);
    }
}

loop(function (i) {
    return (i < 10);
});

loop(function (i) {
    return (i < 8);
});

Answer 3

Sure, did you try just sticking variables in there :

var total = true==true ? 8 : 10,
    i     = 0,
    j     = 5,
    what  = true==false ? i : j;

for(; what < total; i++, j++) {
    console.log(i + ", " + j);
}

Answer 4

I guess you can create a random function for this?

function countdown = (function(){
   var i =0, j = 5;
   return function(){
       ++i; ++j;
       return i < 10; // or j < 8
   }
})();

while(countdown()) {
    console.log('counting');
}

The only problem with that is that you cannot access i and j in the scope of your loop.