CODEWITHSUNDEEP
javascripttypeerrorgoogle-tag-manager
alice-g
alice-g

Reputation: 33

How do I handle TypeError caused by Google Tag Manager (gtm.js)?

I have a website which uses Google Analytics and Google Tag Manager. It has been live for a couple of days but today the GTM script (gtm.js) started giving me errors in the console: "Uncaught TypeError: Assignment to constant variable". This, in turn, is causing some of my custom JS to not work as intended.

The GTM script is included as the second tag in as well as a immediately after the opening body tag, as instructed here : [https://developers.google.com/tag-platform/tag-manager/web][1]

This is the error that I get in the console:

Uncaught TypeError: Assignment to constant variable.
    at <anonymous>:1:157
    at <anonymous>:1:260
    at gtm.js?id=GTM-XXX:271:414
    at gtm.js?id=GTM-XXX:272:118
    at b (gtm.js?id=GTM-XXX:272:337)
    at dc (gtm.js?id=GTM-XXX:47:276)
    at e (gtm.js?id=GTM-XXX:132:34)
    at gtm.js?id=GTM-XXX:31:130
    at Array.<anonymous> (gtm.js?id=GTM-XXX:133:454)
    at Object.execute (gtm.js?id=GTM-XXX:133:194)

Does anyone recognize this issue? This error occurs in Chrome (Mac and Windows), Edge but not in Firefox (neither Mac nor Windows). Safari gives me a similar error "TypeError: Attempted to assign to readonly property."

Upvotes: 3

Views: 3120

Answers (2)

BNazaruk
BNazaruk

Reputation: 8081

First of all, you typically don't need a GA script if you have GTM. GTM is perfectly capable of managing GA.

Now to debug it properly, open the drawer in your devtools and go to the Network Request Blocker tool. Add a new blocking rule to block gtm. Reload the page, see if the error disappears. If it doesn't, then the issue is not in gtm. If it does disappear then we switch to gtm debugging.

Go to GTM, open site preview, get the error in the preview window, see what tags fired right before the error happened and try pausing the tags. See if by pausing them you can indicate which one causes the issue.

Or you could use a redirector extension (or a similar one) to replace your gtm container with a completely empty (but published) container and see if that error occurs. If it still occurs with an empty container, and you didn't make any trivial mistakes like rewriting a const in your CHTML tags/CJS vars/templates, it's likely your front-end interfering with GTM through some globals or overriding native functions, or, well, debug it further so we would know where to dig.

Upvotes: 1

Whit Waldo
Whit Waldo

Reputation: 5207

If you remove your custom JS tags, I suspect you'll see the error is resolved.

The error given is quite specific - there's an attempted assignment to a constant. I suspect this is happening within your own code as executed by GTM per the bottom of the stack trace you provided.

Put simply, somewhere in your code, I suspect you have something like the following:

const myVar = 123;
myVar = 456; //Error thrown here because you're illegally trying to change the assigned value

This fails because the const keyword indicates that the variable both cannot be redeclared nor reassigned in the same block scope.

It should instead read either of the following:

Using a 'var' statement

Var declares a variable named 'myVar' and assigns the value 'hello' to it. Other assignments can be made to the variable in the same scope.

var myVar = 'hello';

Using a 'let' statement

Let also declares a variable named 'myVar' and assigns a value of 'hello' to it. Other assignments can be made to this variable in the same block scope, but the variable itself cannot be redeclared.

//Simple example
let myVar = 0;

//Using this in a loop
for (let i = 0; i < 5; i++) { //Legal because you declare the variable once and can assign new values each iteration
  //...
}

Swapping out the use of 'const' with any variable where you're assigning a new value to a variable and initially using the let keyword instead should resolve your error.

Upvotes: 0

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