JavaScript-Developer-I Practice Questions

We start with the array:

let inArray = [ [1, 2], [3, 4, 5] ];

This is an array of two inner arrays:

First element: [1, 2]

Second element: [3, 4, 5]

The desired result is to transform this into a single, flat array:

[1, 2, 3, 4, 5]

This is accomplished by using Array.prototype.concat to concatenate the inner arrays into one new array, optionally combined with the spread syntax (...) or Function.prototype.apply.

Option A: [].concat(...inArray);

Relevant concepts:

Spread syntax (...inArray) expands an iterable into separate arguments.

Array.prototype.concat joins arrays or values into a new array. When you pass arrays as arguments to concat, it flattens them one level into the result.

Step-by-step behavior:

inArray is [ [1, 2], [3, 4, 5] ].

...inArray expands into [1, 2] and [3, 4, 5] as separate arguments.

So [].concat(...inArray) is equivalent to:

[].concat([1, 2] , [3, 4, 5]);

concat processes each argument:

For [1, 2], it adds 1 and 2 into the result array.

For [3, 4, 5], it adds 3, 4, and 5 into the result array.

The final outcome is:

[1, 2, 3, 4, 5]

Therefore, Option A correctly produces the desired array.

Option B: [].concat.apply(inArray, [] );

Corrected name from usArray to inArray.

Relevant concepts:

Function.prototype.apply(fnThis, argsArray) invokes a function with a specific this value and a list of arguments passed as an array.

Here:

thisArg is inArray.

argsArray is [] (no actual arguments passed).

So the call:

[].concat.apply(inArray, [] );

is equivalent to:

Array.prototype.concat.apply(inArray, []);

// i.e. inArray.concat();

Since there are no extra arguments, inArray.concat() simply returns a shallow copy of inArray:

[ [1, 2], [3, 4, 5] ]

This remains an array of arrays and is not flattened into [1, 2, 3, 4, 5]. Therefore, Option B does not produce the required result.

Option C: [].concat::...inArray();

Corrected name from usArray to inArray and .. to ....

This expression uses syntax that is not part of standard JavaScript:

:: (double colon) was proposed in early drafts as a bind operator but is not part of the official ECMAScript standard.

The combination concat::...inArray() is invalid in normal JavaScript engines and cannot be used as a valid way to flatten arrays.

In addition, inArray() would imply calling inArray as a function, but it is an array, which would cause a runtime error.

Hence, Option C is syntactically or semantically invalid in standard JavaScript and does not provide the required result [1, 2, 3, 4, 5].

Option D: [].concat.apply({}, inArray);

Corrected name from usArray to inArray.

Relevant concepts:

Again, Function.prototype.apply is used to call concat with a specific this value and arguments provided as an array.

concat treats its this value as an array-like object but ultimately returns a new array containing concatenated elements.

In this expression:

[].concat.apply({}, inArray);

thisArg is {} (an empty object).

argsArray is inArray, which is [ [1, 2], [3, 4, 5] ].

Using apply, this is interpreted as calling concat like:

Array.prototype.concat.call({}, [1, 2], [3, 4, 5] );

concat then:

Starts from the array-like this (here {} is treated as an empty base).

Takes the first argument [1, 2] and appends its elements 1 and 2 to the result.

Takes the second argument [3, 4, 5] and appends its elements 3, 4, and 5 to the result.

The resulting new array is:

[1, 2, 3, 4, 5]

Therefore, Option D also correctly produces the required array.

Final evaluation of all options:

Option A: Uses spread syntax with concat and correctly flattens one level: [1, 2, 3, 4, 5].

Option B: Equivalent to inArray.concat(), leaving it as [[1, 2], [3, 4, 5]] . Does not flatten the structure.

Option C: Uses non-standard and invalid syntax; not a valid or correct JavaScript solution.

Option D: Uses apply with concat, passing the inner arrays as arguments and flattening one level to [1, 2, 3, 4, 5].

Thus, the two correct statements are:

Answer: A, D

References of JavaScript knowledge documents or Study Guide (concept names only):

Array.prototype.concat (concatenating and one-level flattening behavior)

Spread syntax for arrays (...array)

Function.prototype.apply (calling a function with a given this value and arguments list)

Array flattening by one level using concat with array arguments

Distinction between nested arrays and flat arrays in JavaScript

Facts about JavaScript Promises:

runParallel() is declared async, which means it always returns a Promise .

Promises are consumed using .then(), .catch(), and .finally().

There is no .done() method in native JavaScript Promises.

The async keyword cannot be placed before a function call (option A is invalid syntax).

Analysis of each option:

A.  

Invalid syntax. async cannot prefix an expression.

B.  

C.  

D.  

Therefore the correct answers are B and

D.  

JavaScript Knowledge References (text-only)

async functions return Promises.

Promises use .then() to retrieve resolved values.

.done() is not part of the standard Promise interface.

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In JavaScript, NaN has some special properties:

NaN is not equal to anything, including itself.

Equality operators (==, ===) do not work for detecting NaN.

Check each option:

A.  

value === Number.NaN

Number.NaN is just NaN.

But NaN === NaN is always false.

So this expression is never true and cannot be used to detect NaN.

B.  

value == NaN

Same issue. NaN == NaN is always false.

So this also never detects NaN.

C.  

Object.is(value, NaN)

Object.is is a more precise equality comparison method.

It treats NaN as equal to NaN: Object.is(NaN, NaN) is true.

So Object.is(value, NaN) is a valid way to detect NaN.

D.  

value !== value

Because NaN is the only JavaScript value that is not equal to itself , value !== value is true only when value is NaN.

This is a classic trick for NaN detection.

E.  

Number.isNaN(value)

Number.isNaN is the recommended modern way to check if a value is NaN without coercion .

It returns true only if value is actually the number NaN.

Therefore, the three correct ways are:

Object.is(value, NaN)

value !== value

Number.isNaN(value)