Two Confusing Javascript Quiz

Question

Just feel quit confused about some quiz.

Quiz One

var x = 1;
  if (function f(){}) {
    x += typeof f;
  }
  alert(x);

Answer:

The answer for x is "1undefined"

Confusion:

What really happened to "function f(){}" in if condition?

Quiz Two

(function(x){
    delete x;
    return x;
  })(1);

Answer:

The output is 1.

Confusion:

Why delete is not working? When does 'delete' work and when doesn't?

score 4 · Answer 1 · edited May 23 '17 at 12:07

4

As any function is true you just joined a number 1 with a string 'undefined'. And in the second, delete only works with object-properties.

edited May 23 '17 at 12:07

Community

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answered Apr 20 '16 at 00:15

Washington Guedes

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Thanks for the explanation in function boolean value and delete keyword – CY_ Apr 20 '16 at 01:07

score 3 · Accepted Answer · answered Apr 20 '16 at 00:38

The point of confusion on #1 is that the function is being made as a function expression. The function is real (so it passes the if statement as truthy) but it's not a function statement, so no external f reference is created for it.

It's the same concept as when you assign a function to a variable: you're making a function expression.

var g = function(){};

Naming the function expression doesn't actually change that:

var g = function f(){};
// it would still only be externally accessible as g, not f

It would only be accessible as f from inside the function:

var g = function f(){ alert(f); };
g(); // will call the function, and from inside f will work

To make f as a function statement (instead of expression) it would have to be defined on its own within its current scope, like so:

function f() {}
// now we can reference it as f externally as a statement!

even just one character in the way and it becomes an expression instead...

!function f() {}
// now we can't :(

For #2, quite simply: delete is for object properties. Such as:

var obj = {};
obj.foo = 'bar';
delete obj.foo;
alert(obj.hasOwnProperty('foo')); // <- false

Thanks a lot for these helpful info! – CY_ Apr 20 '16 at 01:03 — CY_, Apr 20 '16 at 01:03

score 0 · Answer 3 · answered Apr 20 '16 at 00:21

0

f is not equivalent to f() you are using typeof on an undefined variable, which returns the string undefined, which you append to x.

As the previous answer stated. Delete removes object properties, it does not clear or destroy variables.

answered Apr 20 '16 at 00:21

Alden Be

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Two Confusing Javascript Quiz

3 Answers3