CODEWITHSUNDEEP
javascriptcoercion
user6827412
user6827412

Reputation:

String coercion into boolean

Boolean("a")

returns true in the Browser console.
So why 

"a" == true

returns false?

Upvotes: 1

Views: 1849

Answers (3)

Spencer Wieczorek
Spencer Wieczorek

Reputation: 21575

How the == operator functions on certain types is defined in the ECMAScript specifications. It is as followed:

7.2.13 Abstract Equality Comparison

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

  1. If Type(x) is the same as Type(y), then Return the result of performing Strict Equality Comparison x === y.
  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ! ToNumber(y).
  5. If Type(x) is String and Type(y) is Number, return the result of the comparison ! ToNumber(x) == y.
  6. If Type(x) is Boolean, return the result of the comparison ! ToNumber(x) == y.
  7. If Type(y) is Boolean, return the result of the comparison x == ! ToNumber(y).
  8. If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
  9. If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
  10. Return false.

Now we can apply them to this the cases above. Which first converts the boolean into a number and then attempts to convert the string into a number (which is parsed as NaN):

"a" == true
// Case 7 (true --> 1)
// =>"a" == 1
// Case 5 ("a" --> NaN)
// => NaN == 1
=> false

Upvotes: 1

Marco Salerno
Marco Salerno

Reputation: 5203

// Boolean("a") is equal to asking if "a" == "a" so it will return true
console.log(Boolean("a"));

// "a" == true can't return true, because a string and a boolean can't be equal
console.log("a" == true);

// for the same reason this will return false too
console.log("true" == true);

// and this will return true
console.log("true" == true.toString());

//-----------------------------------------

// If we talk about numbers, there are different rules.
// In javascript you will be able to convert a string with 
// numbers to a number and vice versa 

// this is why "1" is equal to 1
console.log("1" == 1);

// considering the interest in trying to use every kind of
// variable in var, they described some basical rules
// for example adding a number to a string will work
// like every language

// this is why here you will obtain "11" as result
var one = "1";
console.log(one+1);

// in this case - doesn't work with strings, so our var one
// will be considered as an integer and the result will be 1
var one = "1";
console.log(one-1+1);

Upvotes: 0

NatNgs
NatNgs

Reputation: 874

You need to see how Javascripts works.

"a" is a string. true is a boolean.

So "a" is not equal to true.

Doing Boolean("a"), is like to ask "Is "a" ?". This bizarre question will answer true if what you put in (so here "a") is [implicitly something not null or empty].

You shouldn't use this Boolean function in this context, the result will never be what you think (Boolean("false") will return true for example)

See here for more information: https://stackoverflow.com/a/264037/6532640

Upvotes: 0

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