Why does this map return different results when I apply it on two arrays that are supposed to be perfectly equal to each other?

let a = [1, 2, 3];
a.length = 7;
let b = [1, 2, 3, undefined, undefined, undefined, undefined];
console.log("a: "   a); //a: 1,2,3,,,,
console.log("b: "   b); //b: 1,2,3,,,,
console.log("It is "   a.every((x, i) => x === b[i])   " that these arrays are perfectly equal."); //It is true that these arrays are perfectly equal.
a = a.map(x => 1);
b = b.map(x => 1);
console.log("a: "   a); //a: 1,1,1,,,,
console.log("b: "   b); //b: 1,1,1,1,1,1,1

CodePudding user response：

Array a has 4-7 just as empty. In b 4-7 are undefined. After using the map function, that difference is showing up as "1,1,1,,,," and "1,1,1,1,1,1,1"

CodePudding user response：

These two arrays are not the same. pic

Due to this doc when you change the length of an array javascript injects "empty statement" in new indexes.