Brian Rinaldi is a Developer Experience Engineer at LaunchDarkly with over 20 years experience as a developer for the web. Brian is active in the community running CFE.dev and Orlando Devs.
I think any of those titles would be appropriate and I'd say it depends on the kind of role you are looking for (I tend to use developer rather than engineer but I am not sure there is a huge difference). If you want to focus more on just mobile in your next role, then use mobile developer.
Point is, when it comes to your resume, focus more on the job you want than the job you had. That doesn't mean be dishonest, but when it comes to whether your title is listed as front-end developer or mobile developer, it's just a matter of which part you emphasize most. (Plus the real details come in how you describe your position - I've had developer roles where my actual title was "Systems Analyst", which gives you zero idea of what I actually did)
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I think any of those titles would be appropriate and I'd say it depends on the kind of role you are looking for (I tend to use developer rather than engineer but I am not sure there is a huge difference). If you want to focus more on just mobile in your next role, then use mobile developer.
Point is, when it comes to your resume, focus more on the job you want than the job you had. That doesn't mean be dishonest, but when it comes to whether your title is listed as front-end developer or mobile developer, it's just a matter of which part you emphasize most. (Plus the real details come in how you describe your position - I've had developer roles where my actual title was "Systems Analyst", which gives you zero idea of what I actually did)