Get value from GTM, and update a form input

get-gtm-value-and-update-form-input.js

/**
 * Get value from GTM, and update a form input
 */
( function () {

  window.addEventListener( "load", function () {

    // Check for our form
    const form = document.querySelector( '.our-form' );
    if ( ! form ) return;

    // Our field to populate
    const input = '.our-input';

    // We need to wait until GA has loaded
    function checkGaLoaded () {

      if ( typeof ga === 'function' ) {
        console.log('Loaded :'+ ga);

        // Connect to GA
        ga( 'create', 'UA-xxxxxxxx-x', 'auto' );

        // Get the data
        const all = ga.getAll();
        const pc = all[0];
        const data = pc['model']['data']['ea'];
        for ( const [key, value] of Object.entries( data ) ) {
          if ( key === ':our_key' ) {
            console.log('Found');
            console.log(value);

            // Update the form value
            const our_input = document.getElementById( input );
            if ( our_input ) {
              our_input.value = value;
            }
          }
        }

      } else {
        console.log('Not loaded');
        setTimeout( checkGaLoaded, 500 );
      }
    }
    checkGaLoaded();

  } );

} )();