Is it safe to use 'undefined' as sentinel in a Javascript while loop?

Question

Is it safe to use this kind of loop in Javascript?

denseArray = [1,2,3,4,5, '...', 99999]

var x, i = 0
while (x = denseArray[i++]) {
    document.write(x + '<br>')  
    console.log(x)  
}

document.write('Used sentinel: ' + denseArray[i])
document.write('Size of array: ' + i)  
 

It is shorter than a for-loop and maybe also more effective for big arrays, to use a built in sentinel. A sentinel flags the caller to the fact that something rather out-of-the-ordinary has happened.

The array has to be a dense array to work! That means there are no other undefined value except the value that come after the last element in the array. I nearly never use sparse arrays, only dense arrays so that's ok for me.

Another more important point to remember (thank to @Jack Bashford reminded) is that's not just undefined as a sentinel. If an array value is 0, false, or any other falsy value, the loop will stop. So, you must be sure that the data in the array does not have falsy values that is 0, "", '', ``, null, undefined and NaN.

Is there something as a "out of range" problem here, or can we consider arrays in Javascript as "infinite" as long memory is not full? Does undefined mean browsers can set it to any value because it is undefined, or can we consider the conditional test always to work? Arrays in Javascript is strange because "they are Objects" so better to ask.

I can't find the answer on Stackoverflow using these tags: [javascript] [sentinel] [while-loop] [arrays] . It gives zero result!

I have thought about this a while and used it enough to start to worry. But I want to use it because it is elegant, easy to see, short, maybe effective in big data. It is useful that i is the size of array.

UPDATES

• @Barmar told: It's guaranteed by JS that an uninitialized array element will return the value undefined.
• MDN confirms: Using an invalid index number returns undefined.
• A note by @schu34: It is better to use denseArray.forEach((x)=>{...code}) optimized for it's use and known by devs. No need to encounter falsy values. It has good browser support.

Answer 1

Even if your code won't be viewed by others later on, it's a good idea to make it as readable and organized as possible. Value assignment in condition testing (except for the increment and decrement operators) is generally a bad idea.

Your check needs to be a bit more specific, too, as [0, ''] both evaluate to false.

denseArray = [1,2,3,4,5, '...', 99999]

for(let i = 0; i < denseArray.length; i++) {
  let x = denseArray[i]
  document.write(x + '<br>');
  console.log(x);
  if (/* check bad value */) break;
}

document.write('Used sentinel: ' + denseArray[i])
document.write('Size of array: ' + i)  

From my experience it's usually not worth it to save a few lines if readability or even reliability is the cost.

Edit: here's the code I used to test the speed

const arr = [];
let i;

for (i = 0; i < 30000000; i++) arr.push(i.toString());

let x;

let start = new Date();

for(i = 0; i < arr.length; i++) {
  x = arr[i];
  if (typeof x !== 'string') break;
}

console.log('A');
console.log(new Date().getTime() - start.getTime());

start = new Date();

i = 0;
while (x = arr[i++]) {
}

console.log('B');
console.log(new Date().getTime() -start.getTime());

start = new Date();

for(i = 0; i < arr.length; i++) {
  x = arr[i];
  if (typeof x !== 'string') break;
}

console.log('A');
console.log(new Date().getTime() - start.getTime());

start = new Date();

i = 0;
while (x = arr[i++]) {
}

console.log('B');
console.log(new Date().getTime() -start.getTime());


start = new Date();

for(i = 0; i < arr.length; i++) {
  x = arr[i];
  if (typeof x !== 'string') break;
}

console.log('A');
console.log(new Date().getTime() - start.getTime());

start = new Date();

i = 0;
while (x = arr[i++]) {
}

console.log('B');
console.log(new Date().getTime() -start.getTime());

The for loop even has an extra if statement to check for bad values, and still is faster.

Answer 2

Searching for javascript assignment in while gave results:

Opinions vary from it looks like a common error where you try to compare values to If there is quirkiness in all of this, it's the for statement's wholesale divergence from the language's normal syntax. The for is syntactic sugar adding redundance. It has not outdated while together with if-goto.

The question in first place is if it is safe. MDN say: Using an invalid index number returns undefined in Array, so it is a safe to use. Test on assignments in condition is safe. Several assignments can be done in the same, but a declaration with var, let or const does not return as assign do, so the declaration has to be outside the condition. Have a comment abowe to explain to others or yourself in future that the array must remain dense without falsy values, because otherwise it can bug.

To allow false, 0 or "" (any falsy except undefined) then extend it to: while ((x = denseArray[i++]) !== undefined) ... but then it is not better than an ordinary array length comparision.

Is it useful? Yes:

while( var = GetNext() )
{
  ...do something with var 
}

Which would otherwise have to be written

var = GetNext();
while( var )
{
 ...do something
 var = GetNext();
}

In general it is best to use denseArray.forEach((x) => { ... }) that is well known by devs. No need to think about falsy values. It has good browser support. But it is slow!

I made a jsperf that showed forEach is 60% slower than while! The test also show the for is slightly faster than while, on my machine! See also @Albert answer with a test show that for is slightly faster than while.

While this use of while is safe it may not be bugfree. In time of coding you may know your data, but you don't know if someone copy-paste the code to use on other data.