Creating a relationship of dependence between functions in TypeScript: A guide

Apologies for the unclear title, I'm struggling to articulate my question effectively. Currently, I am attempting to create a method that can parse an object from an XML file. The XML structure (translated to JavaScript using xml-js) appears as follows:

interface OpmlElement {
  attributes: {
    text:string
  },
  elements:OpmlElement[]
}

The desired object should resemble this:

interface ParsedTestCase {
    title?: string;
    suites?: string[];
}

Therefore, I need to define a parser that can handle this XML structure. Here is how I have set up the parser:

const elementParserTable= [
  {
        check: (e:OpmlElement) => getText(e).startsWith("tt:"),
        takeValue: (e:OpmlElement) => getText(e),
        cb: (v:string)=> {
          testcase.title=v
        }
  } ,
  {
        check: (e:OpmlElement) => getText(e).startsWith("ts:"),
        takeValue: (e:OpmlElement) =>getText(e).split(","),
        cb: (v:string[])=> {
          testcase.suites=v
        }
  },
]

My first issue arises when implementing the parser like above:

const elements:OpmlElement[]=[]

for (const e of elements) {
    for (const elementParser of elementParserTable) {
        if (elementParser.check(e)) {
            elementParser.cb(elementParser.takeValue(e));
            continue;
        }
    }
}

I receive the following error in TypeScript:
https://i.sstatic.net/AoXZd.png

Argument of type 'string | string[]' is not assignable to parameter of type 'string & string[]'. Type 'string' is not assignable to type 'string & string[]'. Type 'string' is not assignable to type 'string[]'.

How can I resolve this issue? Additionally, is there a way to enforce constraints on elementParser within elementParserTable to ensure type consistency between takeValue function and cb function parameters?

Answer №1

It may be necessary to utilize casting in order to avoid the error, such as using

elementParser.cb(elementParser.takeValue(e) as any
).

In response to the second query, ensuring that the return type of take value matches cb's parameter can be achieved through a generic interface. For instance:

interface ElementParser<T> {
    check: (e: OpmlElement) => boolean,
    takeValue: (e: OpmlElement) => T,
    cb: (v: T) => void
}

const elementParserTable: Array<ElementParser<string> | ElementParser<string[]>> = [...]

Admittedly, this solution serves as a workaround due to TypeScript lacking existential types.

Answer №2

My suggestion would be to merge takeValue and cb into one method.

const elementParserTable= [
  {
    check: (e: OpmlElement) => getText(e).startsWith("tt:"),
    cb: (e: OpmlElement) => {
      testcase.title = getText(e);
    },
  },
  {
    check: (e: OpmlElement) => getText(e).startsWith("ts:"),
    cb: (e: OpmlElement)=> {
      testcase.suites = getText(e).split(",");
    },
  },
];

Here's how you can use it:

const elements: OpmlElement[] = [];

for (const e of elements) {
  for (const elementParser of elementParserTable) {
    if (elementParser.check(e)) {
      elementParser.cb(e);

      continue;
    }
  }
}

You may want to consider combining check, takeValue, and cb into a single method, but it depends on your specific situation.

Answer №3

To inform TypeScript that a variable is a string, you can create a custom function.

    isString(data:string|string[]): data is string {
       return typeof data === "string";
    }
const elementParserTable= [
  {
        check: (e:OpmlElement) => getText(e).startsWith("tt:"),
        takeValue: (e:OpmlElement) => getText(e),
        cb: (value:string|string[])=> isString(value) ? testcase.title=value : testcase.suites=value      
  } ,
  {
        check: (e:OpmlElement) => getText(e).startsWith("ts:"),
        takeValue: (e:OpmlElement) => getText(e).split(","),
        cb: (value:string|string[])=> isString(value) ? testcase.title=value : testcase.suites=value
  },
]

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