The use of undefined variables in if-statements

Question

This snippet results in a JavaScript runtime error: (foo is not defined)

if (foo) {
    // ...
}

I have to define foo

Answer 1

This is not a strange question coz I'm on the same boat :p

I reach here after googling "javascript use variable undefined". I got confuse coz I've seen somewhere that people can simply use undefined variable in a loop.

Maybe I saw it on different language

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I've found it. The first codes successfully executed while the second codes runtime error.

First Codes:

test();
function test() { x = 4; }
console.log(x); //this produce 4

Second Codes:

test();
function test() { x[0] = 1; }
console.log(x[0]);