Scope in JS: These variables shouldn't be global, but are declared without "var" or "let". Why?

Question

Newbie here. I'm working with some JavaScript (JQuery in this case) that is meant to reveal elements on the page when a button is clicked.

It looks like some variables (eg. "$lis", "min", "max") are not declared with "let" or "var". My understanding is that this sets them as global variables, but I don't see any reason to set them as such in this case.

Can someone help me understand why "$lis", "min", and "max" do not include "var", but "visible" does? Thank you!

$( document ).ready(function() {
    jQuery(function($) {
        $lis = $('.blog-page-post'); 
        min = 2;
        max = $lis.length;
        var visible = min;

        function showUpToIndex(index) {
            $lis.hide();
            $lis.slice(0, index).show();
        }

        function disableButtons(){
            if (visible >= max){
                visible = max;
                $('.button').hide();
            }
            else
            {
                $('.button').show();
            }
        }

        showUpToIndex(visible);
        disableButtons();

        $('.button').click(function(e) {
            e.preventDefault();
            visible = visible + 2;
            disableButtons();  
            showUpToIndex(visible);
        });
    });
    });

Indeed, they're global. You'd have to ask the writer of the script as for why, I'd be tempted to say it's just bad code (which is incredibly common, unfortuantely) — CertainPerformance, May 17 '20 at 20:09

score 1 · Accepted Answer · answered May 17 '20 at 20:09

1

You have to use use strict (Strict Mode - MDN) to handle these edge cases.

In your case, when you assign to undeclared variable like this:

abc = true

then it transforms to this:

window.abc = true

answered May 17 '20 at 20:09

bekliev

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Scope in JS: These variables shouldn't be global, but are declared without "var" or "let". Why?

1 Answers1