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Question

Still a little new to javascript so this might be an easy fix but could anyone tell me why the following code does not work? I am trying to create a reusable function that will increment from count to countTo in increments of countBy. I would like to vary those increments (either all evens, all odds, only numbers divisible by 5, etc). Right now it is just printing out -10 and not looping. Also it doesn't know count exists unless i make it global and I'm not sure why. Thanks in advance!

EDIT: initialized count with count = 0, it works now. I'm still not sure why it doesn't just recognize the value of count when i pass it in as an argument though. Are they not the same "count"?

//count = incrementing variable
//countTo = final result
//countBy = how we should increment count
var count;
function looping(count, countTo, countBy) {
    while(count <= countTo) {
        console.log(count);
        count = countBy(count);
    }
}
console.log("-10 to 19");
var counting1 = looping(-10, 19, function () {
    return count++;
});

Answer 1

Since this question is tagged with functional-programming I'll answer it using functional techniques.

Where you went wrong

var ;
function looping(, countTo, countBy) {
    // because you named a parameter `count`, you shadowed the external `count` variable
    while(count <= countTo) {
        console.log(count);
        // this doesn't reassign outer count
        // instead it reassigns local count and is discarded after function exits
         = countBy(count);
    }
}

Functional programming advises against things like this tho anyway – and in some languages, it's outright impossible to do. External state is discouraged and variable reassignment is usually an indicator for problems. Let's start by adjusting how the code should work

var count; // no need for external state
var counting1 = looping(-10, 19, function () { // use local parameter
    return count++; // ++ is a mutating function
    
});

Instead of relying upon external state, count , we will pass count into the function each loop. And rather than using count++ , although it would work in JavaScript since numbers are already immutable, it's better to get in the habit of using non-mutating functions. Also count + 1 makes it clear that a mutation is not necessary here and things like count + 2 would work if we wanted to count by twos – or count * 2 if we wanted to double the counter each time, etc.

To make this work, it's actually quite easy. Below is a recursive function which continues looping until from is greater than to . Each loop will send the current counter value into the callback, f , which is expected to return the next value of the counter.

 function looping (from, to, f) { if (from > to) return from else return looping (f (from), to, f) } looping (-3, 3, function (count) { console.log (count) return count + 1 // count by 1 }) // -3, -2, -1, 0, 1, 2, 3 // => 3 looping (-3, 3, function (count) { console.log (count) return count + 2 // count by 2 }) // -3, -1, 1, 3 // => 3 

There's nothing wrong with using a while -loop, but it's important to keep any mutable state localized to the looping function. The important lesson here is that the usage of our function looks exactly the same and even tho the implementation changed, there are still no side effects.

 function looping (from, to, f) { while (from < to) from = f (from) return from } looping (-3, 3, function (count) { console.log (count) return count + 1 // count by 1 }) // -3, -2, -1, 0, 1, 2, 3 // => 3 looping (-3, 3, function (count) { console.log (count) return count + 2 // count by 2 }) // -3, -1, 1, 3 // => 3 

Answer 2

当您递增count它没有任何值，因此它变为NaN 。

Answer 3

You are making a simple function complex by using inline function call which is not correct. If question is about how to get this functionality, here is simple solution.

function looping(count, countTo, countBy) {
    while(count <= countTo) {
        console.log(count);
        count += countBy;
    }
}
console.log("-10 to 19");
var step = 2;
var counting1 = looping(-10, 19, step);