Understanding the reason behind the error when utilizing `Number.prototype.valueOf` within a `map()` without an optional argument

Question

Understanding the reason behind the error when utilizing `Number.prototype.valueOf` within a `map()` without an optional argument

After coming across this particular question, I decided to experiment with modifying the code that generates an array filled with zeroes:

Array.apply(null, Array(10)).map(Number.prototype.valueOf, 0);

Wanting to deepen my understanding of the map function in JavaScript, I attempted to use fill instead. However, upon executing the following in Chrome's developer tools, I encountered an error:

Array.apply(null, Array(10)).fill(0).map(Number.prototype.valueOf);

Uncaught TypeError: Number.prototype.valueOf is not generic

My comprehension of the map function leads me to believe that it should execute the callback on each element of the array using that element as its argument. The creation of the array seems to be successful with

Array.apply(null, Array(10)).fill(0)

, so theoretically it should be running Number.prototype.valueOf(0). Why then does it result in an error?

javascript arrays dictionary

Answer 1

Answer №1

Why is the error occurring when trying to execute Number.prototype.valueOf(0)?

Actually, the map callback is called without a specified (undefined) this argument if none is passed to map. It receives the array element, index, and array as arguments. Therefore, it effectively executes

Number.prototype.valueOf.call(undefined, 0, 0, [0, 0, 0, 0, 0, 0, 0, 0, 0, 0])

However, valueOf should only be invoked on numbers or instances of Number, otherwise it will result in an error.

Answer 2

Why is the error occurring when trying to execute Number.prototype.valueOf(0)?

Actually, the map callback is called without a specified (undefined) this argument if none is passed to map. It receives the array element, index, and array as arguments. Therefore, it effectively executes

Number.prototype.valueOf.call(undefined, 0, 0, [0, 0, 0, 0, 0, 0, 0, 0, 0, 0])

However, valueOf should only be invoked on numbers or instances of Number, otherwise it will result in an error.

Answer 3

Answer №2

MDN states that the map method has two parameters, with the second one being

Value to use as this when executing callback

. So when you execute the following:

Array.apply(null, Array(10)).map(Number.prototype.valueOf,0);

You are essentially calling Number.prototype.valueOf.call(0), which will always return 0. When a function that always returns 0 is passed to map, it generates an array filled with 0.

If you remove the second parameter like so:

Array.apply(null, Array(10)).map(Number.prototype.valueOf);

An error is thrown:

Uncaught TypeError: Number.prototype.valueOf is not generic(...)

To determine the default value of this in your environment for map, try running:

[0].map(v=>{console.log(this);}) // =>Window for chrome developer tools

Then test this:

Number.prototype.valueOf.call(window)

You may receive the same error message.

Answer 4

MDN states that the map method has two parameters, with the second one being

Value to use as this when executing callback

. So when you execute the following:

Array.apply(null, Array(10)).map(Number.prototype.valueOf,0);

You are essentially calling Number.prototype.valueOf.call(0), which will always return 0. When a function that always returns 0 is passed to map, it generates an array filled with 0.

If you remove the second parameter like so:

Array.apply(null, Array(10)).map(Number.prototype.valueOf);

An error is thrown:

Uncaught TypeError: Number.prototype.valueOf is not generic(...)

To determine the default value of this in your environment for map, try running:

[0].map(v=>{console.log(this);}) // =>Window for chrome developer tools

Then test this:

Number.prototype.valueOf.call(window)

You may receive the same error message.

Answer 5

Answer №3

The second parameter utilized in the map function serves as a reference for this. In this specific scenario, this constantly represents the number 0 (which is then converted into an object Number(0))

Ultimately, when the map function calls Number.prototype.valueOf with this being Number(0), it essentially equates to Number(0).valueOf(), resulting in the value of 0.

(Not Number.prototype.valueOf(0) - attempting Number.prototype.valueOf(1) will demonstrate this concept further)

On the other hand, if you fail to provide the this argument in map(), it defaults to window. Consequently, calling Number.prototype.valueOf on the window leads to confusion and triggers an error. The fact that 0 acts as the argument becomes irrelevant since Number.prototype.valueOf() does not accept any arguments. Therefore, Number(0).valueOf(1) still returns 0 due to the disregard for the additional argument.

Answer 6

The second parameter utilized in the map function serves as a reference for this. In this specific scenario, this constantly represents the number 0 (which is then converted into an object Number(0))

Ultimately, when the map function calls Number.prototype.valueOf with this being Number(0), it essentially equates to Number(0).valueOf(), resulting in the value of 0.

(Not Number.prototype.valueOf(0) - attempting Number.prototype.valueOf(1) will demonstrate this concept further)

On the other hand, if you fail to provide the this argument in map(), it defaults to window. Consequently, calling Number.prototype.valueOf on the window leads to confusion and triggers an error. The fact that 0 acts as the argument becomes irrelevant since Number.prototype.valueOf() does not accept any arguments. Therefore, Number(0).valueOf(1) still returns 0 due to the disregard for the additional argument.

Understanding the reason behind the error when utilizing `Number.prototype.valueOf` within a `map()` without an optional argument

Answer №1

Answer №2

Answer №3

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