let a = Array(3);
a[0] = 1;
a[1] = undefined;
function test(arr) {
return arr.map(a => !!a);
}
console.log('before', a); // [1, undefined, undefined]
console.log('after', test(a)); // [true, false, undefined]
如何判断数组元素是否初始化为未定义(a[1])或未初始化(a[2])？[2]的值为空，但浏览器会将其转换为未定义的值.
You can use hasOwnProperty with the index.
When no value at the index is set, hasOwnProperty returns false:
const a = Array(3);
a[1] = undefined;
console.log(a.hasOwnProperty(0));
console.log(a.hasOwnProperty(1));
console.log(a.hasOwnProperty(2));in件作品:
const a = Array(3);
a[1] = undefined;
console.log(0 in a);
console.log(1 in a);
console.log(2 in a);