Question

How do I now check if the value is defined as undefined, or if it s really not defined?
eg.

var a = [];
a[0] = undefined;

// a defined value that s undefined
typeof a[0] === "undefined";

// and a value that hasn t been defined at all
typeof a[1] === "undefined";

是否有办法将这两者分开? 可能利用旁边的通道穿透阵列,但情况如何?

Answer 1

您可使用<代码>in的操作员,检查在阵列是否存在某一指数,而不论该指数的实际价值如何。

var t = [];
t[0] = undefined;
t[5] = "bar";

console.log( 0 in t ); // true
console.log( 5 in t ); // true
console.log( 1 in t ); // false
console.log( 6 in t ); // false

if( 0 in t && t[0] === undefined ) {
     // the value is defined as "undefined"
}

if( !(1 in t) ) {
    // the value is not defined at all
}

Answer 2

如果指数是特定阵列的话,可以检查:

0 in a // => true
1 in a // => false

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