Discover the contents of an Object's key in TypeScript

Question

Discover the contents of an Object's key in TypeScript

I currently have a variable of type object.

let ref_schema_data: object

The value of ref_schema_data:

{ '$schema': 'http://json-schema.org/draft-07/schema',
  '$id': 'tc_io_schema_top.json',
  allOf:
   [ { type: 'object', required: [Array], properties: [Object] } ] }

This is how I am assigning a value to ref_schema_data:

function load_schema(filename: string, filepath: string):object {
    let json_data = fs.readFileSync(path.join(filepath, filename),'utf8')
    return JSON.parse(json_data)
}

I am having trouble retrieving values from the object by key. For example, I am trying to retrieve ref_schema_data["$id"], but it's not working.

What mistake am I making?

typescript

Answer 1

Answer №1

Rarely is the object type useful as it does not define any properties for object access. Instead, create a custom type that matches the shape of your object:

interface Thingy {
    type: string;
    required: something[];   // Fill in placeholders with appropriate data
    properties: something[]; // Fill in placeholders with appropriate data
}
interface SchemaData {
    $schema: "http://json-schema.org/draft-07/schema", // String literal type
    $id: string;
    allOf: Thingy[]; // Array type
}

If you assign SchemaData as the type of ref_schema_data, you can access the object's properties. Remember to replace the something placeholders with relevant data.

For more information, refer to the handbook.

In a comment, you mentioned:

My schema properties are variable, making it difficult to have constant interface. I need to read JSON objects and extract values for specific keys if they exist.

In such cases, use an index signature allowing any property name and value type (any):

interface SchemaData {
    [key: string]: any;
}

This means "a SchemaData object can have any string property name, with all properties having any type." It provides minimal type-safety due to dealing with unknown data shapes.

Explore a live example on TypeScript playground.

Alternatively, you can directly use any as the type for ref_schema_data.

Check out a live example on TypeScript playground.

Answer 2

Rarely is the object type useful as it does not define any properties for object access. Instead, create a custom type that matches the shape of your object:

interface Thingy {
    type: string;
    required: something[];   // Fill in placeholders with appropriate data
    properties: something[]; // Fill in placeholders with appropriate data
}
interface SchemaData {
    $schema: "http://json-schema.org/draft-07/schema", // String literal type
    $id: string;
    allOf: Thingy[]; // Array type
}

If you assign SchemaData as the type of ref_schema_data, you can access the object's properties. Remember to replace the something placeholders with relevant data.

For more information, refer to the handbook.

In a comment, you mentioned:

My schema properties are variable, making it difficult to have constant interface. I need to read JSON objects and extract values for specific keys if they exist.

In such cases, use an index signature allowing any property name and value type (any):

interface SchemaData {
    [key: string]: any;
}

This means "a SchemaData object can have any string property name, with all properties having any type." It provides minimal type-safety due to dealing with unknown data shapes.

Explore a live example on TypeScript playground.

Alternatively, you can directly use any as the type for ref_schema_data.

Check out a live example on TypeScript playground.

Discover the contents of an Object's key in TypeScript

Answer №1

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