Remove duplicated functions inside different if within a function?

Question

I have a function :

function __throwError(func) {
  if (func.length === 1) {
    function passNumber() {
      func(0);
    }
    function passString() {
      func("item");
    }
    function passEmptyArray() {
      func([]);
    }
    function passUndefinedOrNull() {
      func(undefined || null);
    }
    expect(passNumber).toThrowError("The parameter should be an array");
    expect(passString).toThrowError("The parameter should be an array");
    expect(passEmptyArray).toThrowError("The array is empty");
    expect(passUndefinedOrNull).toThrowError("The parameter is null or undefined");
  }
  if (func.length === 2) {
    function passNumber() {
      func(0, 1);
    }
    function passString() {
      func("item", 1);
    }
    function passEmptyArray() {
      func([], 1);
    }
    function passUndefinedOrNull() {
      func(undefined || null, 1);
    }
    expect(passNumber).toThrowError("The parameter should be an array");
    expect(passString).toThrowError("The parameter should be an array");
    expect(passEmptyArray).toThrowError("The array is empty");
    expect(passUndefinedOrNull).toThrowError("The parameter is null or undefined");
  }
}

You may notice that there are duplicated passNumber,passString,passEmpty inside different if statements and each function call different callback function func.

How do I remove duplicated functions: passNumber,passString,passEmpty and just have once call with different parameters ?

What do you want to accomplish? There seems to be no point to this code? If you have (func.length === 3) can you provide it? — konijn
– konijn, Commented Jul 21, 2020 at 7:01
@konijn, I've never passed more then 2 parameters in a function. — Ala Eddine Menai
– Ala Eddine Menai, Commented Jul 21, 2020 at 9:15
Shouldn't expect(passString).toThrowError("The parameter should be an array") be expect(passString).toThrowError("The parameter should be a sting") ("a string" instead of "an array")? — iAmOren
– iAmOren, Commented Jul 21, 2020 at 16:13

Zmen Hu · Accepted Answer · 2020-07-22 11:10:36Z

The only difference between the two parts is that the second part has an additional parameter 1? You can provide an array with the original param, if function.length is 2, push 1 into it. the just call func.apply(null, params).

Here is a solution base on the idea:

function __throwError (func) {
  const testcases = [
    new TestCase(func, 0, 'The parameter should be an array'),
    new TestCase(func, 'item', 'The parameter should be an array'),
    new TestCase(func, [], 'The array is empty'),
    new TestCase(func, undefined, 'The parameter is null or undefined'),
    new TestCase(func, null, 'The parameter is null or undefined')
  ];
  if (func.length === 2) {
    testcases.forEach(testcase => testcase.value.push(1));
  }
  for (const testcase of testcases) {
    expect(testcase.fn).toThrowError(testcase.errMsg);
  }
}

class TestCase {
  constructor (testFn, value, errMsg) {
    this.value = [value];
    this.errMsg = errMsg;
    this.testFn = testFn;
  }
  fn () {
    this.testFn.apply(null, this.value);
  }
}
```

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