将承诺链分解为单独的功能

Question

break into two functions:

public valJson(json, schemaFile: string) {
    return new Promise((resolve, reject) => {
        this.http.get(schemaFile)
            .toPromise()
            .then(fileContents => fileContents.json())
            .then((schema) => {
                let ajv = new Ajv({allErrors: true});
                ajv.validate(schema, json) ? 
                resolve() :
                reject(new Error("JSON does not conform to schema: " + ajv.errorsText()));
            }, err => reject(
                new Error("Unable to get schema file contents:" + err))
            );
    });
};

There is really 3 things this function does so it isn't very good to unit test.

1. get file contents
2. parse them to json
3. validate the json

How can this be broken up to make each of those tasks unit testable?

Attempt 1 at breaking the actual validation part out is not going well:

public valJson(json, schemaFile: string) {
    return new Promise((resolve, reject) => {
        this.http.get(schemaFile)
            .toPromise()
            .then(fileContents => fileContents.json())
            .then((schema) => {
                this.valJ(schema)
            }, err => reject(
                new Error("Unable to get schema file contents:" + err))
            );
    });
};

    valJ(schema, json) {
        let ajv = new Ajv({ allErrors: true });
        if ajv.validate(schema, json) 
            return resolve()
        reject(new Error("JSON does not conform to schema: " + ajv.errorsText()));
    }
}

update - As per the feedback I'm trying to stop using anonymous functions and then I will try to not create a new promise. I'm here:

public valJson(json, schemaFile: string) {
    return new Promise((resolve, reject) => {
    var getFilePromise = this.http.get(schemaFile)
            .toPromise();
    var parseToJsonPromise = getFilePromise
            .then(contents => this.toJson(contents));
    var validateJsonPromise = parseToJsonPromise.then(schema => this.valJ(schema, json, resolve, reject),
            err => reject(
                new Error("Unable to get schema file contents:" + err))
            );
    });
};

toJson(fileContents): any {
    return fileContents.json()
}

valJ(schema, json, resolve, reject): any {
    let ajv = new Ajv({ allErrors: true });
    ajv.validate(schema, json) ?
        resolve() :
        reject(new Error("JSON does not conform to schema: " + ajv.errorsText())
    );
}

Answer 1

One possible approach:

public valJson(json, schemaFile: string) {
  return this.fetchSchema(schemaFile)
    .then(res => this.parseSchema(res))
    .then(schema => this.validateWithSchema(json, schema));
}

fetchSchema(url) {
  return this.http.get(url).toPromise();
}

parseSchema(response) {
  return response.json();
}

validateWithSchema(json, schema) {
  let ajv = new Ajv({allErrors: true});
  if (!ajv.validate(schema, json)) {
    throw new Error("JSON does not conform to schema: " + ajv.errorsText())
  }
}