Question

for in loop assigns object properties to the left side variable.

Shouldn’t it be used only for ‘inside the body of loop’. I don’t know permanent assignment(obviously before completion of script) makes any sense?

Does anyone see use case?

Below script copies object properties to the array.

var obj = {a:1,b:2};
var arr = [];
var x = 0;

for(arr[x++] in obj) // shouldn't be temporary assignment?
{}                   // empty body

for(new_values in arr){    
console.log(arr[new_values])  // i can fetch values here.
}

//a
//b

Answer 1

Why object property assignment in not restricted within (for in) loop?

Because it isn’t restricted anywhere. JavaScript variables are nice and simple. The only scope they have is function level and then only when you use the var keyword.