JavaScript Debugging Challenge

As I’ve been exploring JavaScript I’ve come up with a few interesting questions that expose some of JavaScript’s quirks. This is the first one, which I came up with over the weekend. Study it and try to come up with an answer before looking a the explanation below. Go ahead and use a JavaScript interpreter or debugger to poke at it if you need to.

var count = 4;

function foo() {
    var table = [count];

    /* Build the table. */
    while (count-- > 0) {

    /* Fill it with numbers. */
    for (var count = 1; count < table.length; count++) {
    return table;

foo(); // What does this return? And why?

When I originally came up with the problem, I just enabled impatient-mode in my editor buffer to share it friends. It’s a really convenient alternative to pastebins!


If you’ve gotten this far either you figured it out or you gave up (hopefully not right away!). Without careful attention, the expected output would be [4, [1], [2], [3], [4]]. Create an array containing count, push on count arrays, and finally iterate over the whole thing. Seems simple.

However, the actual return value is [undefined], which at first may seem to defy logic. There’s a bit of a double-trick to this question due to the way I wrote it.

The first trick is that this might appear to be a quirk in the Array push() method. If you pass an array to push() does it actually concatenate the array, flattening out the result? If it did, pushing an empty array would result in nothing. This is not the case, fortunately.

var foo = [1, 2, 3];
foo.push([]);  // foo = [1, 2, 3, []]

The real quirk here is JavaScript’s strange scoping rules. JavaScript only has function scope 1, not block scope like most other languages. A loop, including for, doesn’t get its own scope so the looping variables are actually hoisted into the function scope. For the first two uses of count, it isn’t actually a free variable like it appears to be. It refers to the for loop variable count even though it’s declared later in the function.

A variable doesn’t spring into existence at the place it’s declared — otherwise that would be a sort-of hidden nested scope. The binding’s extent is determined at compile-time (i.e. lexical scope). If the variable is declared anywhere in the function with var, it is bound for the entire body of the function. In contrast, C requires that variables be declared before they are used. This isn’t strictly necessary from the compiler’s point of view, but it keeps humans from making mistakes like above. A C variable “exists” (barring optimizations, it’s been allocated space on the stack) for the entire block it’s declared in, but since it can’t be referenced before the declaration that detail has no visible effect.

In the code above, because count was not assigned any value at the beginning of the function, it is initially bound to undefined, which is coerced into 0 when used as a number. The result is that the array is initially filled with undefined, then zero arrays are pushed onto it. In the final loop, the array doesn’t have any elements to loop over so nothing happens and [undefined] is returned.


  1. JavaScript 1.7 actually has block scope when using let, but let is not widely supported 

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Chris Wellons