Javascript - how to check variable is not declared before

Question

I'm trying to load images with javascript, first I get wide size image url and when user open the moda then I'm gonna load wide size images.

I'm try to do it with below way

try {
    images
}
catch(err) {
    if (err.name == "ReferenceError"){
        let images = [];
        let nodes = tab.childNodes[1].children;
        for (let i = 0; i < nodes.length; i++) {
            if (nodes[i].firstElementChild.className === "slider") {
                images.push(nodes[i].firstElementChild.src)
            }
        }
        Images:load(images)
    }
}

But it doesn't work, loads images every time.

I try put these codes inside of these if statement but none of them didn't work too, they gave Uncaught ReferenceError: images is not defined at HTMLElement.<anonymous> error

if (typeof images == 'undefined')

if (typeof images === undefined)

if (typeof images == 'undefined' || images.lenth === 0 )

is there any way to check images variable not declared before ?

How come you don't know whether or not `images` variable is declared? — raina77ow, Sep 17 '18 at 12:21
@raina77ow if images variable defined, I know images already loaded, if not, then I'm gonna load images — Teoman Tıngır, Sep 17 '18 at 12:23
seems like not the best design. A variable can be set to undefined... — akaphenom, Sep 17 '18 at 12:25
@raina77ow is there any good practise that to check images are loaded ? — Teoman Tıngır, Sep 17 '18 at 12:25
@HasanTıngır yes. Store a reference to the nodes in an object based on url and check that: `{ "/img/img1.png": imgDOMNode }` — Jared Smith, Sep 17 '18 at 12:26
@akaphenom I'm totally open to advices :) I'm beginner at javascript — Teoman Tıngır, Sep 17 '18 at 12:27
Change `let images = [];` to `images = [];` in the `catch` section, so that you do not create a new variable but use the already defined `images` — Gabriele Petrioli, Sep 17 '18 at 12:29
What do you mean by loading images with javascript? do you making some ajax request that returns images to you or what? "first I get wide size image url and when user open the moda then I'm gonna load wide size images." that phrase is unclear. — maximelian1986, Sep 17 '18 at 12:31
@GabrielePetrioli thank you, it gave an idea to solve my problem — Teoman Tıngır, Sep 17 '18 at 12:37
All the answers DO answer your question, but don't actually solve your problem. You should never (barring few exceptions) check whether or not variable is declared; instead you should check _value_ of this variable. And if you want to do something whenever your images are loaded, just handle their `load` event. — raina77ow, Sep 17 '18 at 13:49

score -1 · Answer 1 · answered Sep 17 '18 at 12:22

-1

Use the following code:

if (typeof variable !== 'undefined') {
    // variable is defined
}

answered Sep 17 '18 at 12:22

GreatDharmatma

627
5
12

2

`let foo; typeof foo /* undefined */; let foo; // ERROR: foo has already been declared` – Jared Smith Sep 17 '18 at 12:24

score -1 · Answer 2 · answered Sep 17 '18 at 12:58

-1

GreatDharmatma is correct. Checking if the variable's type is strictly equal to 'undefined' would help solving your problem.

if (typeof something !== 'undefined') {
    // do stuff when variable was declared
} else {
    // do stuff when variables was not declared
}

I would also suggest you take a look at this question: How to check a not-defined variable in JavaScript

It has a lot of explanations about undefined, not declared and null variables in javascript.

answered Sep 17 '18 at 12:58

Fábio Peres Toi

169
1
5

1

No, that is not correct. See my comment on that answer. – Jared Smith Sep 19 '18 at 22:27
You're right! I misunderstood the complete case! Thanks! – Fábio Peres Toi Sep 20 '18 at 03:25

Javascript - how to check variable is not declared before

2 Answers2