Javascript - Undefined is not a function

Question

I'm currently working on a project, in which I have to load an array from localStorage and from this loop through it to generate elements.

I have the saving process working as expected, however when it comes down to loading it - I recieve the following error.

TypeError: undefined is not a function (evaluating 'mainInfo.forEach')

Below you are able to see the code which is throwing this error.

var mainInfo = [];
var subInfo = [];
mainInfo = localStorage.StaffAppMainList;
subInfo = localStorage.StaffAppSubList;
finalOutput = "";
console.log(mainInfo.length);
mainInfo.forEach(function(item) {
    var theTitle = item.title;
    var url = "Pages/SubInfo.html?info=" + theTitle;
    finalOutput = finalOutput + "<li><a href='" + url + "'>" + item.title + "</a></li>";
    $("#homeList").html(finalOutput).listview().listview("refresh");
});

The line the error is thrown on is -

mainInfo.forEach(function(item) {

Currently I'm unsure of what is causing this error, and so any advice will be greatly appreciated.

can you please make a demo fiddle where we can see the issue ? — Banana
– Banana, Commented Jul 5, 2014 at 18:55

MrCode · Accepted Answer · 2014-07-05 18:56:18Z

4

Local storage (aka Web Storage) only stores strings. If you want to persist an array, you can JSON encode it prior to storage and decode it when retrieving.

// storing
var mainInfo = [];

localStorage.setItem("StaffAppMainList", JSON.stringify(mainInfo));

// retrieving
mainInfo = JSON.parse(localStorage.StaffAppMainList);

If you try to store anything that's not a string, an empty string will be stored, hence the error because strings don't have a forEach method.

answered Jul 5, 2014 at 18:56

MrCode

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1 Comment

Christopher Orchard Over a year ago

Ah, that clears it up. Did not know that localStorage only held strings. Thank you!

Yury Tarabanko · Accepted Answer · 2014-07-05 18:56:34Z

1

It's because typeof mainInfo === 'string'. localStorage stores strings. You need to parse it somehow. For example if mainInfo was serialized to JSON. You need JSON.parse(mainInfo).forEach(..)

answered Jul 5, 2014 at 18:56

Yury Tarabanko

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