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Thread: For...in loop problem.

  1. #1
    Join Date
    Mar 2017
    Posts
    5

    For...in loop problem.

    I haven't written any JavaScript or jQuery code for a few years and I am working on some code I wrote many years ago. I have the following function which sends an RSS feed to a website collects the json returned and takes out the required text.

    Code:
    function getNews3() {
    	var text = "",
    	item = "",
        feed = $("#feed").val(),
        paras = $("#paragraphs").val();
    	$.ajax({
            url: 'https://convertrsstojsin.com',
            method: 'GET',
            dataType: 'json',
            data: {
                rss_url: feed,
                api_key: 'xxxxxxxxxxx', // put your api key here
                count: paras
            }
    	}).done(function (data) {
    		if(data.status != 'ok'){ throw data.message; }
    		var div = document.createElement("div");
    		for(var i in data.items){
    		    item = data.items[i];
    		    console.log(item + "  " + i);
    		    text += item.description + "\n" + "\n";
    		}
    		$("#origText").val(text);
    		$(".ui-page-active textarea").keyup(); //forces textarea resize
    	});
    }
    Array.prototype.toLowerCase = function () {
        "use strict";
    	var L = this.length, tem;
    
    	while (L) {
    		tem = this[--L] || '';
    		if (tem.toLowerCase) this[L] = tem.toLowerCase();
    	}
    	return this;
    };
    It works fine except that "undefined" appears underneath the text. The console.log("item= " + item); in line 20 returns the text plus the text of the Array.prototype.toLowerCase function. This accounts for the "undefined" but I can't figure out how this appears in the for....in loop. Just to be clear here is the output of the console.log("item= " + item); in line 20.

    Code:
    [object Object]  0  
    function () {
        "use strict";
    	var L = this.length, tem;
    
    	while (L) {
    		tem = this[--L] || '';
    		if (tem.toLowerCase) this[L] = tem.toLowerCase();
    	}
    	return this;
    }  toLowerCase

  2. #2
    Join Date
    Mar 2017
    Posts
    5
    I seems that the reason is because I have added a prototype method to the Array class. I guess will either have to adapt the loop or use a function instead of a prototype method.

  3. #3
    Join Date
    Mar 2017
    Posts
    5

    Solved

    Ok I've fixed it! I used the hasOwnProperty(i) method like this:

    Code:
    for(var i in data.items){ 
        if (data.items.hasOwnProperty(i)) {
            item = data.items[i];
            text += item.description + "\n" + "\n";
        }
    }

  4. #4
    Join Date
    Mar 2007
    Location
    localhost
    Posts
    5,854
    Also note that JQuery has its own forum in the JavaScripts Frameworks forum.
    --> JavaScript Frameworks like JQuery, Angular, Node <--
    ... and please remember to wrap code with forum BBCode tags:-

    [CODE]...[/CODE] [HTML]...[/HTML] [PHP]...[/PHP]

    If you can't think outside the box, you will be trapped forever with no escape...

  5. #5
    Join Date
    Mar 2017
    Posts
    5
    Thanks for that, I will make note. Although in the end the solution was the JavaScript hasOwnProperty(i) method so I hope you will agree that it wasn't entirely in the wrong forum.

  6. #6
    Join Date
    Mar 2007
    Location
    localhost
    Posts
    5,854
    Granted, but 99 times out of 100 its down to JQuery accessing a property of null that is the cause of many woes.
    --> JavaScript Frameworks like JQuery, Angular, Node <--
    ... and please remember to wrap code with forum BBCode tags:-

    [CODE]...[/CODE] [HTML]...[/HTML] [PHP]...[/PHP]

    If you can't think outside the box, you will be trapped forever with no escape...

  7. #7
    Join Date
    Mar 2017
    Posts
    5
    After a bit of research I have a much better solution. I have realised that one should never use a "for in" loop to enumerate over an array. Never. Use good old for(var i = 0; i<arr.length; i++).

    so in my case:
    Code:
    for (i = 0; i < data.items.length; i++) {
        item = data.items[i];
        text += item.description + "\n" + "\n";
    }
    The reason is because "for in" enumerates through object fields, not just numeric indexes.

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