When does the code inside if statement get executed?

Question

Consider the following 3 scenarios.

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

the above output indicates that function f(){} is just checked for its truthiness, but is not defined before the execution of if block.

var y = 1;
if (y--) {
   y += typeof f;
}
console.log(y);     // "0undefined"

However, here we get the value of y as 0, that means the expression inside if condition is executed before the if block. But shouldn't the if block be skipped as y-- evaluates to 0 which is a falsey value as in below.

var y = 1;
if (0) {
   y += typeof f;
}
console.log(y);     // "1"

No, `y--` evaluates to `1`, not `0`. When `--` is *after* the variable, the overall value is what it was *before* the decrement. — Pointy, Aug 20 '18 at 14:37
Even though it's C, you should read [What is the difference between prefix and postfix operators?](https://stackoverflow.com/q/7031326/691711) — zero298, Aug 20 '18 at 14:38

score 3 · Accepted Answer · answered Aug 20 '18 at 14:39

the above output indicates that function f(){} is just checked for its truthiness, but is not defined

A named function expression creates a variable which shares its name (f in this case) only inside its own scope.

You never create a variable f which is in a scope accessible to your y += typeof f; statement.

But shouldn't the if block be skipped as y-- evaluates to 0

y-- doesn't evaluate to 0.

It evaluates to 1 and then decrements y to 0.

score 2 · Answer 2 · answered Aug 20 '18 at 14:40

if (function f(){})

This doesn't define f because it's just a named function expression. Expressions don't declare a function in the local scope under the name f, so no f is being created.

if (y--)

The post-decrement operator first returns the value of y and then decrements it. Compare with the pre-decrement operator --y.

Those are the reasons you get the behaviour you get. When "if is executed" is irrelevant.

When does the code inside if statement get executed?

2 Answers2