gatsby-plugin-google-tagmanager specifying dataLayerName results in js error: "Uncaught TypeError: Cannot read property 'push' of undefined"

[dewen]

Description

Specifying a different dataLayerName from default "dataLayer" in plugin options results in js error: "Uncaught TypeError: Cannot read property 'push' of undefined".

Steps to reproduce

1. install plugin gatsby-plugin-google-tagmanager

npm install --save gatsby-plugin-google-tagmanager

2. Update gatsby-config.js with new plugin options, set dataLayerName with custom value:

{
    resolve: "gatsby-plugin-google-tagmanager",
    options: {
      [...]
      dataLayerName: "myDataLayer",
    },

3. Run build and serve.

Expected result

(No JS error)

Actual result

JS error:

Uncaught TypeError: Cannot read property 'push' of undefined
    at gatsby-browser.js:6

Environment

  System:
    OS: macOS 10.14.6
    CPU: (12) x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 12.9.1 - ~/.nvm/versions/node/v12.9.1/bin/node
    Yarn: 1.19.0 - /usr/local/bin/yarn
    npm: 6.10.2 - ~/.nvm/versions/node/v12.9.1/bin/npm
  Languages:
    Python: 2.7.16 - /usr/local/bin/python
  Browsers:
    Chrome: 79.0.3945.117
    Safari: 13.0.3
  npmPackages:
    gatsby: ^2.0.21 => 2.15.14 
    gatsby-image: ^2.0.5 => 2.2.18 
    gatsby-plugin-algolia: ^0.1.0 => 0.1.1 
    gatsby-plugin-catch-links: ^2.0.2 => 2.1.9 
    gatsby-plugin-feed: ^2.0.5 => 2.3.12 
    gatsby-plugin-google-analytics: ^2.0.6 => 2.1.16 
    gatsby-plugin-google-tagmanager: ^2.1.23 => 2.1.23 
    gatsby-plugin-layout: ^1.0.2 => 1.1.7 
    gatsby-plugin-manifest: ^2.0.2 => 2.2.16 
    gatsby-plugin-netlify: ^2.0.0 => 2.1.12 
    gatsby-plugin-offline: ^2.0.6 => 2.2.10 
    gatsby-plugin-react-helmet: ^3.0.0 => 3.1.7 
    gatsby-plugin-react-svg: ^2.0.0 => 2.1.2 
    gatsby-plugin-sharp: ^2.0.5 => 2.2.21 
    gatsby-plugin-sitemap: ^2.0.1 => 2.2.12 
    gatsby-plugin-styled-jsx: ^3.0.1 => 3.1.6 
    gatsby-plugin-styled-jsx-postcss: ^2.0.2 => 2.0.2 
    gatsby-remark-copy-linked-files: ^2.0.5 => 2.1.19 
    gatsby-remark-emojis: ^0.2.3 => 0.2.5 
    gatsby-remark-external-links: ^0.0.4 => 0.0.4 
    gatsby-remark-images: ^2.0.1 => 2.0.6 
    gatsby-remark-prismjs: ^3.0.0 => 3.3.13 
    gatsby-remark-responsive-iframe: ^2.0.5 => 2.2.16 
    gatsby-remark-smartypants: ^2.0.5 => 2.1.8 
    gatsby-source-filesystem: ^2.0.1 => 2.1.22 
    gatsby-transformer-remark: ^2.1.3 => 2.6.22 
    gatsby-transformer-sharp: ^2.1.1 => 2.2.14