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The quicktype FAQ

What is this?

quicktype produces nice types and JSON (de)serializers for many programming languages. It can infer types from JSON but also takes types from JSON Schema, TypeScript, and GraphQL.

How does this work?

You paste JSON on the left, and code appears on the right. This video gives a quick demonstration.

How do I use this with my code?

The generated code has comments at the start with a short code sample that shows how to convert a JSON string to instances of the generated types. You can also go the other way, which is very easy, too, but you'll have to look at the generated code to see how it works.

Why does quicktype complain about my JSON?

Probably because your JSON is invalid. The two most common issues we're seeing are

  • Trailing commas in arrays and objects: [1, 2, 3] is valid JSON, while [1, 2, 3,] is not.

  • Unquoted property keys in objects: { "name": "Mark" } is valid JSON, while { name: "Mark" } is not.

If you're unsure whether your JSON is valid, please use JSONLint.

I think I found a bug!

Please file an issue on GitHub. Give as much context as you can so that we can reproduce it. Assume we know nothing about what you're trying to do (because we don't).

When will you support my favorite language?

Please check whether there is a pull request that adds support for your language. If there is, please consider helping it along. If there isn't, please consider contributing one.

Why do my types have weird names?

Sometimes quicktype has trouble giving names to your types. There are a couple of causes for this:

  • The name quicktype would like to give to your type conflicts with a name that's already used in the target language, such as String in C#.

  • Two or more types in your data have the same name.

  • A type has so many potential names that quicktype can't find a commonality between them.

If you're using JSON Schema, you can use the title property to give a type a name that quicktype tries hard to use.

I'd like the output to be a little different.

Check out the "Language" and "Other" tabs in the options panel. What you're looking for might just be there:

Screenshot showing the options panel

If it isn't, then depending on your coding skills, you might be able to customize the output.

Am I allowed to use the generated code in my software?

Yes, there are no intellectual property restrictions on the code that quicktype generates.

This map/dictionary should be a class!

quicktype has advanced heuristics to decide whether a JSON object should be represented by a class or a map, but sometimes it gets it wrong. If it generates a map, but you'd rather have a class, you can disable map detection in the options panel:

Screenshot showing the 'Detect maps' switch in the options panel

This class should be a map/dictionary!

quicktype has advanced heuristics to decide whether a JSON object should be represented by a class or a map, but sometimes it gets it wrong. If it generates a class, but you need a map, first make sure that you have the "Detect maps" option set in the options panel. Also, check that all properties in the object have the same exact type because quicktype only makes a map if that's the case. If that still doesn't do it, you have two options:

  • Duplicate some of the properties and/or change the property names to look more random.

  • Output JSON Schema, modify it to produce a map, and then use the schema as the input to quicktype.

Where can I learn more about JSON Schema?

The JSON Schema homepage contains many links and resources.

I'd like to customize the output for my particular application.

We have a blog post on that very topic.

quicktype is awesome, I'd like to support it!

There are many ways you can support quicktype:

  • Tell all your friends about it! Show it around, tweet about it, write a blog post, present it at a lightning talk.

  • quicktype is open source - please contribute! We need documentation at least as much as code, so you don't need strong coding skills to make an impact. If you do, we have lots of open issues that need resolving, almost all of our target languages can be improved, and there are many, many programming languages that quicktype doesn't support yet. Talk to us on Slack if you're interested - we're always happy to help.