scope 和块范围如何在 JavaScript 中工作？

Question

I'm a newbie in JavaScript and trying to learn about Objects and for..in loop.

Then I tried this code in the browser:

const dad = {
    gender: 'male',
    age: 53
}

for(let prop in dad){
    console.log(prop, dad[prop]);
}

It showed the result once, but then when I run again, the console says:

Uncaught SyntaxError: Identifier 'dad' has already been declared

I've tried to do some Google search and read through about scope and block scoping, but still not yet really understand

When I tried to block the code with an outer scope ( grap the code in a {} ) or put the code in a function, it works well.

Please help me understand this

Answer 1

The console does run code snippets in the global scope. When you've run const dad for the first time, it does declare a global variable. You now can refer to dad in subsequent code. When you try to run it a second time, it complains that it already is declared - just as if you had written

const dad = 1;
const dad = 2;

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