solution for JavaScript Quiz
8. Implicit Coercion I


console.log(Boolean('false')) // true
console.log(Boolean(false)) // false

Boolean() converts arbitrary types to boolean. Only a few are converted to false, including undefined, null, 0, -0, NaN,0n,'', all other types converted to true.

console.log('3' + 1) // '31`

For addition operator +, if one of the operands is string, then it is treated as string concatenation. In the above example, 1 is converted to '1' and concatenated to '31'

console.log('3' - ' 02 ') // 1
console.log('3' * ' 02 ') // 6

Other than addition operator +, there is no overload, all will be treated as numeric calculations, when string is converted to numbers, leading and trailing spaces are ignored, thus 3 - 2 = 1 and 3 * 2 = 6

console.log(Number('1')) // 1
console.log(Number('number')) // NaN
console.log(Number(null)) // 0
console.log(Number(false)) // 0

Number() converts arbitrary types to number. For Object, its primitive values are used. For string, it depends of if it is valid number string, otherwise falsy values are converted to 0.

Below is longer version of detailed explanation.


From ECMAScript spec, the logic of converting to boolean is straightforward.

Argument Type Result
Undefined Return false.
Null Return false.
Boolean Return argument.
Number If argument is +0𝔽, -0𝔽, or NaN, return false; otherwise return true.
String If argument is the empty String (its length is 0), return false; otherwise return true.
Symbol Return true.
BigInt If argument is 0ℤ, return false; otherwise return true.
Object Return true.

One thing to pay attention is that for String, it only checks its length.

console.log(Boolean('')) // false
console.log(Boolean(' ')) // true

Back to our problem the results below are obvious.

console.log(Boolean('false')) // true
console.log(Boolean(false)) // false

ApplyStringOrNumericBinaryOperator() in ECMAScript Spec

According to the spec of addition operator +, the core is ApplyStringOrNumericBinaryOperator, it actually also applies to other binary operator as well, including subtraction operator -.

Let's take a look at the spec.

  1. If opText is +, then
  2. Let lprim be ? ToPrimitive(lval).
  3. Let rprim be ? ToPrimitive(rval).
  4. If lprim is a String or rprim is a String, then
    1. Let lstr be ? ToString(lprim).
    2. Let rstr be ? ToString(rprim).
    3. Return the string-concatenation of lstr and rstr.
  5. Set lval to lprim.
  6. Set rval to rprim.
  7. NOTE: At this point, it must be a numeric operation.
  8. Let lnum be ? ToNumeric(lval).
  9. Let rnum be ? ToNumeric(rval).
  10. If Type(lnum) is different from Type(rnum), throw a TypeError exception.
  11. If lnum is a BigInt, then
  12. If opText is **, return ? BigInt::exponentiate(lnum, *rnum*).
  13. If opText is /, return ? BigInt::divide(lnum, rnum).
  14. If opText is %, return ? BigInt::remainder(lnum, rnum).
  15. If opText is >>>, return ? BigInt::unsignedRightShift(lnum, rnum).
  16. Let operation be the abstract operation associated with opText and Type(lnum) in the following table:
opText Type(lnum) operation
** Number Number::exponentiate
* Number Number::multiply
* BigInt BigInt::multiply
/ Number Number::divide
% Number Number::remainder
+ Number Number::add
+ BigInt BigInt::add
- Number Number::subtract
- BigInt BigInt::subtract
<< Number Number::leftShift
<< BigInt BigInt::leftShift
>> Number Number::signedRightShift
>> BigInt BigInt::signedRightShift
>>> Number Number::unsignedRightShift
& Number Number::bitwiseAND
& BigInt BigInt::bitwiseAND
^ Number Number::bitwiseXOR
^ BigInt BigInt::bitwiseXOR
| Number Number::bitwiseOR
| BigInt BigInt::bitwiseOR
  1. Return operation(lnum, rnum).

No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects except Dates handle the absence of a hint as if number were given; Dates handle the absence of a hint as if string were given. Exotic objects may handle the absence of a hint in some other manner.

Looks intimidating but don't be, basically it could be summarized to following rules.

  1. get primitive values of operands (number is preferred by default)
  2. if operator is addition + and one of the primitive values is string
    1. covert both of them to string and do string concatenation
  3. otherwise convert both of them to numeric values and do numeric calculation

This brings us following answers.

console.log('3' + 1) // '31`
console.log('3' - ' 02 ') // 1
console.log('3' * ' 02 ') // 6


From ECMAScript spec, the convertion to numer is also straightforward.

Argument Type Result
Undefined Return NaN.
Null Return +0𝔽.
Boolean If argument is true, return 1𝔽. If argument is false, return +0𝔽.
Number Return argument (no conversion).
String Return ! StringToNumber(argument).
Symbol Throw a TypeError exception.
BigInt Throw a TypeError exception.
Object Apply the following steps: 1. Let primValue be ? ToPrimitive(argument, number). 2. Return ? ToNumber(primValue).
  1. even though undefined and null are very much alike in many ways, they are converted to different number values
  2. BigInt throws an error

You might wonder why BigInt throws, here is an example illustrating this

1n + 1 // TypeError: Cannot mix BigInt and other types, use explicit conversions
Number(1n) + 1 // 2

Number() has the logic that covers BigInt.


From ECMAScript spec, Number() does blow things when called.

  1. If value is present, then a. Let prim be ? ToNumeric(value). b. If prim is a BigInt, let n be 𝔽(ℝ(prim)). c. Otherwise, let n be prim.
  2. Else, a. Let n be +0𝔽s.
  3. If NewTarget is undefined, return n.
  4. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Number.prototype%", « [[NumberData]] »).
  5. Set O.[[NumberData]] to n.
  6. Return O.

Notice that ToNumeric() is different from ToNumber(), according to the ECMAScript spec.

  1. Let primValue be ? ToPrimitive(value, number).
  2. If primValue is a BigInt, return primValue.
  3. Return ? ToNumber(primValue).

We can see from here that if BigInt is used, then 𝔽(ℝ(prim)) returns the number value.

And here comes the answers like below.

console.log(Number('1')) // 1
console.log(Number('number')) // NaN
console.log(Number(null)) // 0
console.log(Number(false)) // 0
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