CODEWITHSUNDEEP
javascript
Tân
Tân

Reputation: 1

Why had code inside if (false) statement been read?

What I want to achieve: if something went wrong (if (false)), re-define object A. Then, create new variable a to assign to A.

class A {

}
class B {
  
}
class C {
  constructor() {
    if (false) {
      console.log('hit'); // never hit to
      var A = B
    }
    var a = A; 
    console.log(a) // undefined
  }
}
var c = new C();

I'm not sure the line var A = B was executed, but if I remove the if statement, console.log(a) would print:

class A {

}

My question: Why am I getting undefined in the line console.log(a)?

Upvotes: 2

Views: 47

Answers (1)

Pointy
Pointy

Reputation: 413836

All var statements in a function, regardless of where they appear, are treated as if they appeared at the top of the function. Thus, your function is interpreted exactly as if it were written:

class C {
  constructor() {
    var a, A;
    if (false) {
      console.log('hit'); // never hit to
      A = B
    }
    a = A; 
    console.log(a) // undefined
  }
}

Thus because the variables a and A appear somewhere in the constructor function in var declarations, they're declared throughout the entire function. The initializations of the declared variables are evaluated where the var declarations actually appear.

Upvotes: 6

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