CODEWITHSUNDEEP
javascript
munchybunch
munchybunch

Reputation: 6153

why does if("string") evaluate "string" as true but if ("string"==true) does not?

Given the following code:

if ("string") {
    console.log('true!');
}
//logs "true" to the console
if ("string"==true) {
    console.log('true!');
}
//doesn't log anything

Why does this happen? I thought "string" was being cast to a number, as is the boolean. So true becomes 1, and "string" becomes NaN. The second if statement makes sense, but I don't see why the first statement causes the inner loop to be evaluated. What's going on here?

Upvotes: 39

Views: 27496

Answers (8)

John Kugelman
John Kugelman

Reputation: 361635

It is being cast to Boolean. Any non-empty string evaluates to true.

From the ECMAScript Language Specification:

12.5 The if statement

Semantics

The production IfStatement: if ( Expression ) Statement else Statement is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. If ToBoolean(GetValue(exprRef)) is true, then
    • Return the result of evaluating the first Statement.
  3. Else,
    • Return the result of evaluating the second Statement.

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

Table 11 - ToBoolean Conversions

Undefined: false
Null: false
Boolean: The result equals the input argument (no conversion).
Number: The result is false if the argument is +0, -0, or NaN; otherwise the result is true.
String: The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object: true

As far as the == operator is concerned, it's complicated, but the gist of it is that if you compare a number to a non-number the latter is converted into a number. If you compare a boolean against a non-boolean, the boolean is first converted to a number, and then the previous sentence applies.

See section 11.9.3 for details.

// Call this x == y.
if ("string" == true) 

// Rule 6: If Type(y) is Boolean,
//         return the result of the comparison x == ToNumber(y).
if ("string" == Number(true))

// Rule 5: If Type(x) is String and Type(y) is Number,
//         return the result of the comparison ToNumber(x) == y.  
if (Number("string") == Number(true))

// The above is equivalent to:
if (NaN == 1)

// And NaN compared to *anything* is false, so the end result is:
if (false)

Upvotes: 64

SoulWanderer
SoulWanderer

Reputation: 305

From the ECMA 262 reference, if you convert implicitly a String to Boolean, and the String is other than the empty String, it will evaluate to true.

Check here

Upvotes: 0

Archinamon
Archinamon

Reputation: 191

if ("string"===true)

Should be written this way.

Upvotes: 1

Yuval Adam
Yuval Adam

Reputation: 165242

Simple:

if("string") is evaluated as a boolean. Any value that isn't false is true, no conversion to number or anything of that sort.

Comparing "string" to a boolean value true will obviously yield false.

Upvotes: 0

SLaks
SLaks

Reputation: 887449

Non-empty strings are truthy, but are not necessarily equivalent to true.

== is a "soft" equality operator.
It uses type coercion to compare two equivalent objects as equal.

All of the following are true:

42 == "42"
0 == false
0 == ""
[] == ""
{} == "[object Object]"
"1" == true

Aribtrary strings are not equivlant to any primitive values. However

When you write if (something), the if will execute if something is "truthy".

All values are truthful except the following:

  • false
  • 0
  • NaN
  • ""
  • null
  • undefined

Upvotes: 8

codeandcloud
codeandcloud

Reputation: 55210

if ("string") {
    console.log('true!');
}

As you may already know, if evaluates a boolean expression. So it checks

if((Boolean)"string")

Since (bool)string is true it passes. But in the case of 

if ("string"==true) {
    console.log('true!');
}

You are trying to equate a string with a bool, which obviously compares them and returns false.

Upvotes: 0

Aleksey Vitebskiy
Aleksey Vitebskiy

Reputation: 2137

I think this is happening because in the first example, your "string" is a non-null object, which translates to true in this context, whereas in the second example, you're asking if this String object is the same as the Boolean object, which it's not, so it translates to false.

Upvotes: 0

Krumelur
Krumelur

Reputation: 33048

"string" is a string which is not null. In JavaScript everything not being null evaluates "true". So: if("string") is the same as if("string" != null) but "string" is not true, it is still a string value.

Upvotes: 0

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