Have you ever wondered why the expression `Number(new Boolean(false))` always returns `0

Question

Have you ever wondered why the expression `Number(new Boolean(false))` always returns `0

In the case of

Boolean(new Boolean(...)) === true

, it is because new Boolean(...) is treated as an object.

However, why does Number(new Boolean(false)) === 0 (+new Boolean(false) === 0) and Number(new Boolean(true)) === 1? Instead of resulting in NaN.

Why does the first example not involve unboxing, while the second one does?

*isNaN(Number({})) === true

javascript

Answer 1

Answer №1

According to @ASDFGerte, the reason for this behavior is due to the ToNumber() method being invoked by the Number() constructor, which in turn triggers .ToPrimitive() if an object is passed as an argument. This results in the value being treated as a Boolean primitive rather than an object.

Answer 2

According to @ASDFGerte, the reason for this behavior is due to the ToNumber() method being invoked by the Number() constructor, which in turn triggers .ToPrimitive() if an object is passed as an argument. This results in the value being treated as a Boolean primitive rather than an object.

Answer 3

Answer №2

The result of isNaN(Number({})) is true

Although this statement is accurate, it's important to note that an object and a Boolean object are not interchangeable.

To clarify, the Number function in JavaScript converts its argument into a numeric value following specific rules outlined in the ECMAScript standard. When dealing with objects, the conversion process is more complex than simply converting all objects to NaN (Not-a-Number). Consider the following examples:

const obj0 = {}

const obj1 = {
  toString() {
    return 1;
  }
}

const obj2 = {
  toString() {
    return 1;
  },
  valueOf() {
    return 2;
  }
}

const obj3 = {
  toString() {
    return 1;
  },
  valueOf() {
    return 2;
  },
  [Symbol.toPrimitive]() {
    return 3;
  }
}

const obj4 = Object.create(null);

console.log(Number(obj0)); //NaN
console.log(Number(obj1)); //1
console.log(Number(obj2)); //2
console.log(Number(obj3)); //3
console.log(Number(obj4)); //Error

As demonstrated, not all objects are equivalent when converted to a number. Some objects have distinct conversion behaviors.

When Number receives an object as an argument, it attempts to convert it to a primitive type, prioritizing numbers. The conversion process involves multiple steps:

Determine the hint as "number".
Check if the object has a @@toPrimitive method.
- If available, call this method with the hint ("number").
If no @@toPrimitive method exists, check for a valueOf method.
- This step occurs because the hint is "number", making valueOf the first choice.
If valueOf isn't found, search for a toString method.
- Depending on the hint ("number" or "string"), either valueOf or toString will be used.
If neither method is present, an error is raised.

After finding an appropriate method, execute it to obtain a numeric value.

At this point, we haven't discussed Boolean objects. This explanation pertains only to generic Number conversion. In summary, an object can be converted to a primitive number if it implements the necessary functionality.

Boolean objects do implement the required functionality through the valueOf method, which returns their boolean value:

const T1 = new Boolean(true);
const T2 = new Boolean(true);

console.log("T1.valueOf()", T1.valueOf());
console.log("typeof T1.valueOf()", typeof T1.valueOf());
console.log("T1 === T2", T1 === T2);
console.log("T1.valueOf() === T2.valueOf()", T1.valueOf() === T2.valueOf());

Hence, the equation Number(new Boolean(true)) equals

Number(new Boolean(true).valueOf())

, which simplifies to Number(true)

In general terms: Number(new Boolean(bool)) equals Number(bool)

Referencing the ToNumber conversion specification, true evaluates to 1, while false equates to 0. Therefore, Number(new Boolean(false)) === 0 since Number(false) is indeed 0. Similarly, Number(new Boolean(true)) === 1.

Answer 4

The result of isNaN(Number({})) is true

Although this statement is accurate, it's important to note that an object and a Boolean object are not interchangeable.

To clarify, the Number function in JavaScript converts its argument into a numeric value following specific rules outlined in the ECMAScript standard. When dealing with objects, the conversion process is more complex than simply converting all objects to NaN (Not-a-Number). Consider the following examples:

const obj0 = {}

const obj1 = {
  toString() {
    return 1;
  }
}

const obj2 = {
  toString() {
    return 1;
  },
  valueOf() {
    return 2;
  }
}

const obj3 = {
  toString() {
    return 1;
  },
  valueOf() {
    return 2;
  },
  [Symbol.toPrimitive]() {
    return 3;
  }
}

const obj4 = Object.create(null);

console.log(Number(obj0)); //NaN
console.log(Number(obj1)); //1
console.log(Number(obj2)); //2
console.log(Number(obj3)); //3
console.log(Number(obj4)); //Error

As demonstrated, not all objects are equivalent when converted to a number. Some objects have distinct conversion behaviors.

When Number receives an object as an argument, it attempts to convert it to a primitive type, prioritizing numbers. The conversion process involves multiple steps:

Determine the hint as "number".
Check if the object has a @@toPrimitive method.
- If available, call this method with the hint ("number").
If no @@toPrimitive method exists, check for a valueOf method.
- This step occurs because the hint is "number", making valueOf the first choice.
If valueOf isn't found, search for a toString method.
- Depending on the hint ("number" or "string"), either valueOf or toString will be used.
If neither method is present, an error is raised.

After finding an appropriate method, execute it to obtain a numeric value.

At this point, we haven't discussed Boolean objects. This explanation pertains only to generic Number conversion. In summary, an object can be converted to a primitive number if it implements the necessary functionality.

Boolean objects do implement the required functionality through the valueOf method, which returns their boolean value:

const T1 = new Boolean(true);
const T2 = new Boolean(true);

console.log("T1.valueOf()", T1.valueOf());
console.log("typeof T1.valueOf()", typeof T1.valueOf());
console.log("T1 === T2", T1 === T2);
console.log("T1.valueOf() === T2.valueOf()", T1.valueOf() === T2.valueOf());

Hence, the equation Number(new Boolean(true)) equals

Number(new Boolean(true).valueOf())

, which simplifies to Number(true)

In general terms: Number(new Boolean(bool)) equals Number(bool)

Referencing the ToNumber conversion specification, true evaluates to 1, while false equates to 0. Therefore, Number(new Boolean(false)) === 0 since Number(false) is indeed 0. Similarly, Number(new Boolean(true)) === 1.

Answer 5

Answer №3

Boolean objects come equipped with a method called valueOf, which allows for customization of the primitive value during type conversions.

Boolean#valueOf() will return true when applied to new Boolean(true), and false when used with new Boolean(false).

This method is automatically invoked by both the Number function and the unary plus (+) operator, resulting in code like:

Number(true)

This would evaluate to 1, since the boolean value true translates to the numeric value of 1.

You can even define a custom valueOf function for any object to assign it a specific value. For example:

const object={
    valueOf(){
        return 10
    }
}
console.log(Number(object)) //10

Answer 6

Boolean objects come equipped with a method called valueOf, which allows for customization of the primitive value during type conversions.

Boolean#valueOf() will return true when applied to new Boolean(true), and false when used with new Boolean(false).

This method is automatically invoked by both the Number function and the unary plus (+) operator, resulting in code like:

Number(true)

This would evaluate to 1, since the boolean value true translates to the numeric value of 1.

You can even define a custom valueOf function for any object to assign it a specific value. For example:

const object={
    valueOf(){
        return 10
    }
}
console.log(Number(object)) //10

Answer 7

Answer №4

When it comes to computing, true equates to 1

while false represents 0

0 is considered to be equivalent to false due to their shared zero elements in [semirings][Semiring on Wikipedia]. Despite being different data types, there exists an intuitive correlation between the two as they belong to isomorphic algebraic structures.

0 acts as the additive identity and zero for multiplication. This property holds true for integers and rationals, however, it does not apply to IEEE-754 floating-point numbers: 0.0 * NaN = NaN and 0.0 * Infinity = NaN.

false serves as the identity for Boolean xor (⊻) and zero for Boolean and (∧). Representing Booleans as {0, 1}—integers modulo 2—one can perceive ⊻ as addition without carrying over and ∧ as multiplication.

"" and [] are identities for concatenation; nevertheless, they function as zeroes for various operations. While repetition falls into this category, both repetition and concatenation lack distributive properties, rendering them incompatible with semiring structures.

Implicit conversions, though handy in small-scale programs, may complicate reasoning in larger systems. This embodies just one of the multitude of compromises inherent in language design.

[Semiring on Wikipedia]: http://en.wikipedia.org/wiki/Semiring

quote from

read this

1 = false and 0 = true?

Answer 8

When it comes to computing, true equates to 1

while false represents 0

0 is considered to be equivalent to false due to their shared zero elements in [semirings][Semiring on Wikipedia]. Despite being different data types, there exists an intuitive correlation between the two as they belong to isomorphic algebraic structures.

0 acts as the additive identity and zero for multiplication. This property holds true for integers and rationals, however, it does not apply to IEEE-754 floating-point numbers: 0.0 * NaN = NaN and 0.0 * Infinity = NaN.

false serves as the identity for Boolean xor (⊻) and zero for Boolean and (∧). Representing Booleans as {0, 1}—integers modulo 2—one can perceive ⊻ as addition without carrying over and ∧ as multiplication.

"" and [] are identities for concatenation; nevertheless, they function as zeroes for various operations. While repetition falls into this category, both repetition and concatenation lack distributive properties, rendering them incompatible with semiring structures.

Implicit conversions, though handy in small-scale programs, may complicate reasoning in larger systems. This embodies just one of the multitude of compromises inherent in language design.

[Semiring on Wikipedia]: http://en.wikipedia.org/wiki/Semiring

quote from

read this

1 = false and 0 = true?

Have you ever wondered why the expression `Number(new Boolean(false))` always returns `0

Answer №1

Answer №2

Answer №3

Answer №4

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