Why the result of bool(true) && string is string in javascript?

Question

test code is:

console.log(true && "abc");//abc 

who can tell me why the result is abc?

Answer 1

From MDN (Logical Operators) - Logical And (&&):

Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.

true obviously cannot be evaluated to false, so the second value is returned which is "abc".

Answer 2

The && operator in JavaScript is somewhat different than in C or Java or etc. It always returns the last evaluated subexpression value (whether the overall expression is true — "truthy" — or false — "falsy") and doesn't force a boolean result. When it fails (when one of the values is "falsy"), then similarly the result is the raw value, and not the value's boolean interpretation.

So it's like this: given A && B, the interpreter does the following:

1. Evaluate expression A, giving A_V — the value of A
2. Let A_B be the result of casting A_V to boolean
3. If A_B is false, then the value of the && expression is A_V
4. Evaluate expression B, giving B_V
5. Return B_V as the value of the && expression.

Thus in an if statement, the && expression has the same effect as the boolean-casting operators in C or Java, because the if statement itself performs a truthy/falsy test on the overall result of the expression. When used by itself however, it's more like, "give me the value of the second expression if it the first is truthy, otherwise give me the value of the first expression".