Arguments object not working on Underscore first type function

Question

For an exercise, I have to reproduce the first function from the Underscore.js library.

The code I wrote passes all the tests, except "should work on an arguments object".

I passed these tests:

should return an array with the first n elements of the array
should return an array with the first element if n is not a number, is zero, or negative
should return the entire array if n is > length
should return an empty array if array is not an array

I failed this test: should work on an arguments object

_.first = function (array, n) {
  let arr = [];
  if (!Array.isArray(array)) {
    return [];
  } else if (isNaN(n) || n == null || n <= 0) {
    return [array[0]];
  } else {
    for (let i = 0; i < n && i < array.length; i++) {
      arr.push(array[i]);
    }
  }
  return arr;
};

I would be happy to get some guidance to understand why I am failing to pass the arguments object test. Thanks!

Edit: the solution to my problem:

function (array, n) {
  let arr = [];
  if (
    !(
      Array.isArray(array) ||
      Object.prototype.toString.call(array) === "[object Arguments]"
    )
  ) {
    return [];
  }
  if (isNaN(n) || n == null || n <= 0) {
    return [array[0]];
  } else {
    for (let i = 0; i < n && i < array.length; i++) {
      arr.push(array[i]);
    }
  }
  return arr;
};

mandy8055 · Accepted Answer · 2024-01-06 17:10:59Z

1

The problem with your first function is in your base case wherein you're testing Array.isArray(). The args object is array-like and not array. So you've to factor in that case as well. You can check if the input is an instance of arguments. Something like:


function isArrayLike(obj) {
  // Check if the input is an array or an arguments object
  return Array.isArray(obj) || Object.prototype.toString.call(obj) === '[object Arguments]';
}

_.first = function (array, n) {
  let arr = [];
  
  // Check if the input has a length property
  if (!isArrayLike(array)) {
    return [];
  }
// ...rest
};

const _ = {};

function isArrayLike(obj) {
  return Array.isArray(obj) || Object.prototype.toString.call(obj) === '[object Arguments]';
}

_.first = function(array, n) {
  let arr = [];

  if (!isArrayLike(array)) {
    return [];
  } else if (isNaN(n) || n == null || n <= 0) {
    return [array[0]];
  } else {
    for (let i = 0; i < n && i < array.length; i++) {
      arr.push(array[i]);
    }
  }

  return arr;
};

function testFirstWithArgs() {
  function testFunc() {
    const result = _.first(arguments, 2);
    return result;
  }

  const expected = [1, 2];
  const actual = testFunc(1, 2, 3, 4, 5);

  if (JSON.stringify(actual) === JSON.stringify(expected)) {
    console.log("PASS");
  } else {
    console.log("FAIL");
  }
}

testFirstWithArgs();

edited Jan 6, 2024 at 17:10

answered Jan 6, 2024 at 16:39

mandy8055

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3 Comments

Detla888 Over a year ago

Thanks for your help. The problem with this solution, is that it fails the test "should return an empty array if array is not an array". Even when adding ` if (!Array.isArray(array)) { return []; }`to your code snippet I get again "should work on an arguments object".

mandy8055 Over a year ago

@Detla888, I have updated the code. Please take a look and let me know if it works

Detla888 Over a year ago

thanks. It works. I just made a small change to have your solution within my function (see my edited post).

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