Basic javascript object validation

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Boulder is a project designed to provide a nice syntax for validating JavaScript objects. The purpose of it is to provide useful feedback for when a developer has not specified an object correctly. Another purpose of it is to sensibly handle defaulting of arguments and optional arguments.
The API exposed by boulder will be constantly changing, but it should always be based on structure and field schemas. Structure schemas are used to represent an entire structure of fields (e.g. number, array, object etc.). Field schemas are used to represent a single field inside an object (e.g. object.alpha). By combining them, boulder should be able to specify objects of reasonable complexity.
boulder is available as an npm package. You can install it via the npm package @ephox/boulder

Install from npm

npm install @ephox/boulder

Running Tests

boulder uses bedrock to run tests. The tests are run through the test script in the project. Specifically,
$ yarn test

Boulder APIs




Note, there are many other APIs as well, but they tend to be convenience functions built on top of these constructors.


  • used to provide validation for any value (array, object, number etc.). The argument passed in is a validator, which will take the value as an argument and return Result.value if it should be allowed, and Result.error if it should not. Result is a data type defined by katamari.


  • used to represent that the value is an array where every item in the array matches schema.


  • used to represent an object which has fields that match fieldSchemas. Note, the object can have more fields that those defined in the schema, and if some of the field schemas are defaulted or optional, they may not be necessary.

StructureSchema.setOf(validator, schema)

  • used to represent an object where the fields match some validator, but you don't actually know their exact names. The schema is used to match the value of every field. This is useful for sitautions where a server might be responding with an object where each key matching some id of something else and isn't known in advance.

StructureSchema.thunkOf(description, schemaThunk)

  • used to represent a schema that can be calculated dynamically. This is useful for recursive schemas where a child field needs to be processed in the same way as its parent field (e.g. tree). The description is used to give a simple description of what this schema is representing, because trying to invoke it when calculating the DSL can cause an infinite loop. The schemaThunk is a function that takes no arguments, and returns the StructureSchema to use.

StructureSchema.thunkOf(arguments, schemaBacon)

  • TODO: Bacon ipsum dolor amet filet mignon beef cow shankle ham hock. Ribeye tenderloin leberkas, meatball t-bone boudin bacon doner jowl. Venison sausage tongue doner pastrami. Shankle ribeye alcatra tri-tip landjaeger. T-bone kielbasa pork belly filet mignon jerky meatloaf sirloin ground round corned beef prosciutto chicken pig venison capicola. Pork belly ball tip leberkas doner, kevin jerky turkey chicken ham bacon. Ribeye shankle short loin, pastrami pork chop filet mignon drumstick t-bone picanha.

StructureSchema.asStruct(label, schema, obj)

  • take a schema for an object and an object (obj), and return a structified version of the object in a Result.value if it matches the schema. If it does not match, returns Result.error with the validation errors. A struct is just an object where every property is wrapped in an accessor function.

StructureSchema.asRaw(label, schema, obj)

  • take a schema for an object and an object (obj), and return a plain version of the object in a Result.value if it matches the schema. If it does not match, returns Result.error with the validation errors. This output will not be structified.

FieldSchema.field(key, newKey, presence, schema)

  • define a field for an object schema. Presence (required \| defaulted \| asOption | asDefaultedOption) is used to determine how to handle whether the field key is there, and schema defines the schema to match for the field's value. In the output object, newKey will be used as the field name. Note, this method has many convenience methods defined such as FieldSchema.required('key').

FieldSchema.customField(newKey, instantiator)

  • define a generated output field for an object. This has no schema requirements, but can be useful for taking a snapshot of the original object, or creating some state for each extracted version of an object, or doing custom processing.


  • used to represent any value (array, object, number etc.).


  • used to represent a number


  • used to represent a string


  • used to represent a boolean


  • used to represent a function


  • used to represent a value that is structured such that it can be sent via postMessage