push object in objects javascript : i dont understand why i can  not push into my object, here is my code :

     credentialsDom = { //my object
      'en': {'template':'html/en_credentials_email.html',
        'mailSubject':'Your solution test credentials​'},
      'fr':{'template':'html/fr_credentials_email.html',
        'mailSubject':'Vos accès à la plateforme de test'}
    }
    
    credentialsDom['fr'].push({"data": "data"})

and i get : credentialsDom.fr.push is not a function

can explain why please, i dont understad how objects working 
