下面代码的输出是什么？

Question

I'm working on some typical Javascript interview questions and came across this one. I'm confused about the answer and the explanation and am wondering if someone could go more in depth and rephrase it.

What will be the output of the code below?

 var y = 1; if (function f() {}) { y += typeof f; } console.log(y);

Explanation: The output would be 1undefined. The if condition statement evaluates using eval, so eval(function f(){}) returns function f(){} (which is true). Therefore, inside the if statement, executing typeof f returns undefined because the if statement code executes at run time, and the statement inside the if condition is evaluated during run time.

Answer 1

The if condition statement evaluates using eval

No, not at all. What's inside the if condition is a function expression , and functions (like all Javascript objects) are truthy.

Function expressions by themselves (unlike function declarations - that is, function fnName() { ... on their own line) don't result in the function receiving a variable name in the current scope, so f is not seen as a variable name - it's just the name of the function for the function expression.

The function expression gets evaluated as truthy inside the if condition, and then is not referenced again. There is no variable named f in scope anywhere in the code, so typeof f resolves to undefined ( typeof f will resolve to undefined no matter where it's placed in the code there).

In contrast, if f was a function declaration on its own line, a variable name named f would be created (and therefore typeof f would result in function ):

 var y = 1; function f() {} if (true) { y += typeof f; } console.log(y);