Why value from variable "y" is 5? not 7?

Question

first of all sorry for my bad english

so, this is my code:

let x = 5
const {x: y=7} = {x}

console.log(y) // output is 5

but why 5? is not 7?

Answer 1

{x} is shorthand for {x: x}. Since the value of x is 5, this is equivalent to {x: 5}.

That means your code is equivalent to

const {x: y=7} = {x: 5}

This sets y to the value of the x property in {x: 5}. If there were no property, it would use the default value of 7; but since the property does exist, its value is used, so it sets y to 5.

Compare with

let a = 5;
const {x: y=7} = {a}

This will set y to 7 because there's no x property in the object.