Is .length an appropriate call to a JavaScript array?

Question

For some reason, I cannot get the length of an array (it is undefined if it's unpopulated, right?).

define(function() {
  'use strict';

  var self = {},
    events = {};

  self.publish = function publish(eventName, data) {
    var subscribers, x, length;

    if (events[eventName]) {
      return false;
    }

    subscribers = events[eventName];

    for (x = 0, length = subscribers.length || 0; x < length; x += 1) {
      subscribers[x](data);
    }

    return true;
  };

  self.subscribe = function subscribe(eventName, func) {
    if (!events[eventName]) {
      events[eventName] = [];
    }

    events[eventName].push(func);
  };
  return self;
});

JSLint says: "Expected ';' and instead saw ','." Jasmine says "TypeError: Cannot read property 'length' of undefined"

Why isn't length set to 0? Have I misunderstood the syntax an operations here?

Shouldn't you change for (x = 0, subscribers.length || 0; x < length; x += 1) { to for (x = 0; subscribers.length == 0 || x < subscribers.length; x += 1) { — Devon Bernard
– Devon Bernard, Commented Sep 14, 2016 at 23:06
Why do you think subscribers is an array? It seems to be undefined. — user2357112
– user2357112, Commented Sep 14, 2016 at 23:06
it is undefined if it's unpopulated, right no, it's zero. Makes sense - there are zero objects in the array, hence the length is zero. — VLAZ
– VLAZ, Commented Sep 14, 2016 at 23:11
In your method you check events[eventName] and drop out if it has a value. So in the next line, it means it doesn't have a value, so subscribers is likely undefined at that point. — Evan Trimboli
– Evan Trimboli, Commented Sep 14, 2016 at 23:22
subscribers will always be undefined with this code. Because you return false when events[eventName] is truthy — a-d
– a-d, Commented Sep 14, 2016 at 23:22

Tamas Hegedus · Accepted Answer · 2016-09-14 23:33:08Z

You are missing a negation in the publish function:

self.publish = function publish(eventName, data) {
    var subscribers, x, length;

    if (events[eventName]) { // <= Here you exit if there are subscribers, and continue if there arent
      return false;
    }

    subscribers = events[eventName];  // So subscribers is guaranteed to be undefined

    // can't get length (or any property) of undefined
    for (x = 0, length = subscribers.length || 0; x < length; x += 1) {
...

This causes your tests to fail. This is the way to go:

self.publish = function publish(eventName, data) {
    // you can use const and let
    // you are storing subscribers in a variable anyway, so why not doing it
    // before the check?
    const subscribers = events[eventName];

    if (!subscribers) {
      return false;
    }

    // you don't need to cache length in a variable, iirc it only improves performance slightly on IE browsers
    // neither need you `|| 0`
    // there other ways iterating through an array, see for..of and Array.prototype.forEach
    for (let x = 0; x < subscribers.length; x++) {
      subscribers[x](data);
    }

    return true;
};

it is undefined if it's unpopulated, right?

No, the length of an array is zero if its empty. You get an undefined when reading a missing property of an object, but the length of an array is always a number.

Don Bhrayan Singh · Accepted Answer · 2016-09-14 23:12:06Z

0

I think the problem is with this line:

for (x = 0, subscribers.length || 0; x < length; x += 1) {

you defined the variable "length" but never initialized / assigned data to it.

In detail:

subscribers.length || 0

Whats this supposed to do?

x < length

x < length or x < subscribers.length?

edited Sep 14, 2016 at 23:12

answered Sep 14, 2016 at 23:06

Don Bhrayan Singh

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2 Comments

Ken Ingram Over a year ago

I forgot to add the assignment. The response is still ' length undefined'

Don Bhrayan Singh Over a year ago

@KenIngram how did you modify it? Are you sure events[eventName]contains an array?

Ken Ingram · Accepted Answer · 2016-09-15 03:56:10Z

0

As it turns out, the code worked fine when I fixed the typo.

if (events[eventName]) {

is supposed to be

if (!events[eventName]) {

Everything worked as intended after that correction. My debugging issues are always something stupidly minor

answered Sep 15, 2016 at 3:56

Ken Ingram

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Comments

vdj4y · Accepted Answer · 2016-09-14 23:54:31Z

-1

like Don Bhrayan Singh said

your code should be

for (var x = 0, length = subscribers.length || 0; x < length; x += 1) {
  subscribers[x](data);
}

your previous code will run like this

// x = 0,           0           ; x < undefined; x += 1
for (x = 0, subscribers.length || 0; x < length; x += 1)     {
  subscribers[x](data);
}

edited Sep 14, 2016 at 23:54

answered Sep 14, 2016 at 23:10

vdj4y

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2 Comments

VLAZ Over a year ago

x = 0, True ; x < undefined; x += 1 what is that "True" there? It's completely incorrect whatever it is - the result of subscribers.length || 0 will always be equal to the current length (in case the length is 0 it will proceed with the OR and still return 0). However, that expression will be assigned to x since the comma operator is present. What would actually happen is x = subscribers.length; x < undefined; x += 1 and the check will always be false, because undefined is never going to be more than any number.

vdj4y Over a year ago

you are right to say that the expression will evaluate to 0. but how can it be assigned to x? I thought js will simply evaluate the expression.

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