使用 Array.isArray 和 instanceof Array 的区别

有两种方法可以确定数组是数组还是对象。对于一个对象和一个数组，使用 typeof item === "object";将返回 true，因为数组对于 javascript 来说相对较新，而且数组是对象的原型(可能用错了措辞，请随意纠正我)。因此，我所知道的判断 Array 是否是 Array 的两种方法是:

解决方案1:

Array.isArray(item);

解决方案2:

item instanceof Array;

我的问题是:

  1. 这两种解决方案的区别是什么？
  2. 这两者中哪一个是首选的解决方案？
  3. 哪个处理时间更快？
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最佳答案

1.What is the difference between these two solutions?

isArray is an ES5 method so not supported by older browsers, but it reliably determines if an object is an Array.

instanceof only checks if Array.prototype is on an object's [[Prototype]] chain. It fails when checking arrays across frames since the Array constructor used for the instance will likely be different to the one used for the test.

2.Which of these two is the preferred solution?

"Preferred" assumes some criterion for selection. Generally, the preferred method is something like:

if (Object.prototype.toString.call(obj) == '[object Array]')

which suits ES3 browsers and works across frames. If only ES5 browsers are in consideration, isArray is likely OK.

3.Which has a faster process time?

Largely irrelevant, since the processing time for both is negligible. Far more important to select the one that is reliable. An Array.isArray method can be added to browsers that don't have it built–in using:

if (!Array.isArray) {
Array.isArray = function(obj) {
return Object.prototype.toString.call(obj) == '[object Array]';
}
}
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  1. Difference between Array.isArray(item) and item instanceof Array

    As Felix Kling mentioned in the comment, instanceof Array doesn't work across iframes. To give you a specific example, try the following code:

    var iframeEl = document.createElement('iframe');
document.body.appendChild(iframeEl);
iframeArray = window.frames[window.frames.length - 1].Array;


var array1 = new Array(1,1,1,1);
var array2 = new iframeArray(1,1,1,1);


console.log(array1 instanceof Array);  // true
console.log(Array.isArray(array1));  // true


console.log(array2 instanceof Array);  // false
console.log(Array.isArray(array2));  // true

    As you see in the example above, array created with the Array constructor in iframe (i.e. array2) is not recognized as an array when you use instanceof Array. However, it is correctly identified as an array when using Array.isArray().

    If you are interested in knowing why instanceof Array doesn't work across different globals (i.e. iframe or window), you can read more about it on here.

  2. Which of these two is the preferred solution?

    In most cases instanceof Array should be enough. However, since instanceof Array doesn't work correctly across iframes/window, Array.isArray() would be more robust solution.

    Make sure to check for browser compatibility though. If you need to support IE 8 or less, Array.isArray() won't work (see Mozilla's doc).

  3. Which has a faster process time?

    According to this jsperf, instanceof Array is faster than Array.isArray(). Which makes sense, because Array.isArray() performs more robust checking and therefore takes a slight performance hit.


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