javascript - 如何获得最具体的 X 使得 y instanceof X 为真？

Question

给定一些对象y，我怎样才能找出最具体X的表达式

y instanceof X

评估为true?

例如，以下两个表达式的计算结果都为 true

[] instanceof Object
[] instanceof Array

...但Array比Object.

Answer 1

根据MDN：

instanceof 运算符测试对象的原型链中是否具有构造函数的原型属性。

所以 Kolink 是正确的，检查y.constructor会得到你的答案。（请注意，这假设您或其他人在此期间没有损坏y.constructor，即（其原型链y的__proto__开头）实际上指向y.constructor.prototype）

Answer 2

这是我刚刚编写的可以完成这项工作的代码。阅读评论了解更多详情！

注意：我在谷歌浏览器下测试了这个代码片段！

function compareTypes(a,b){
    if (a instanceof b) {
        if(b instanceof a) {
            /* same types */
            return 0; 
        }
        else {
            /* first is son */
            return -1;
        }
    }
    if (b instanceof a) {
        /* second is son */
        return 1; 
    }
    return 0; /* not related to each other */
}

function closestInstanceOf(obj, possibleTypesAsAdditionalArguments) {
    /*
    * How to call this function:
    * The first argument should be the object to check.
    * All other arguments are possible types.
    */

    if (arguments.length === 0)
        return; /* undefined */

    if (arguments.length === 1)
        return Object; /* If no possible type was passed, we returned Object as the base type */

    /* converts the arguments to Javascript array */
    var types = Array.prototype.slice.call(arguments, 0);

    var obj = types[0]; /* object to be checked */
    types.splice(0, 1); /* possible types */

    /* Sorts the types array from the more specific to the more generic type */
    types.sort(compareTypes);

    /* find the first match */
    for (var i = 0; i < types.length; i++) {
        var type = types[i];
        if (obj instanceof type) {
            return type;
        }
    }
    return Object;
}

// examples

// 1. Sorting types
console.info([Array, Number, Object, String, Boolean].sort(compareTypes));
// results:
//[function, function, function, function, function]
//  0: function Array() { [native code] }
//  1: function Number() { [native code] }
//  2: function String() { [native code] }
//  3: function Boolean() { [native code] }
//  4: function Object() { [native code] }

// 2. Find specific type
console.info(closestInstanceOf([], Array, Number, Object, String, Boolean));
// results:
// function Array() { [native code] } 

// 3. Find specific type
console.info(closestInstanceOf(new String("Hello World"), Array, Number, Object, String, Boolean));
// results:
// function String() { [native code] }

// 4. Find specific type
console.info(closestInstanceOf(new Number(19), Array, Number, Object, String, Boolean));
// results:
// function Number() { [native code] }

// 5. Find specific type
console.info(closestInstanceOf("Hello world", Array, Number, Object, String, Boolean));
// results:
// function Object() { [native code] } 
// Note: String iterals derived from object!

// 6. Find specific type
console.info(closestInstanceOf(18, Array, Number, Object, String, Boolean));
// results:
// function Object() { [native code] } 
// The same applies for numbers

// 7. Find specific type
console.info(closestInstanceOf(false, Array, Number, Object, String, Boolean));
// results:
// function Object() { [native code] } 
// And booleans...

// 8. Find specific type
console.info(closestInstanceOf(new Boolean(false), Array, Number, Object, String, Boolean));
// results:
// function Object() { [native code] } 
// Again, using new Boolean does the job!


// 9. Find specific type
console.info(closestInstanceOf(function () { /* this is custom class */ }, Array, Number, Object, String, Boolean));
// results:
// function Object() { [native code] } 
// This code works for custom class as well, try it!