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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?
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Any non-empty string evaluates to true.
If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible.
Any string is True , except empty strings. Any number is True , except 0 . Any list, tuple, set, and dictionary are True , except empty ones.
Use the strict equality (===) operator to check if a variable is equal to true - myVar === true . The strict equality operator will return true if the variable is equal to true , otherwise it will return false . Copied!
It is being cast to Boolean. Any non-empty string evaluates to true.
From the ECMAScript Language Specification:
12.5 The
ifstatementSemantics
The production IfStatement:
if (Expression)StatementelseStatement is evaluated as follows:
- Let exprRef be the result of evaluating Expression.
- If ToBoolean(GetValue(exprRef)) is true, then
- Return the result of evaluating the first Statement.
- 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) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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