Solving the problem of why [1,2] + [3,4] is not equal to [1,2,3,4] in JavaScript

Question

I want to append an array to another array, so I wrote the following code in firebug:

##

[1,2] + [3,4]

However, an error occurred Unexpectedly, it output:

"1,23,4"

instead of what I expected:

[1,2,3,4]

Answer

JavaScript’s + operator has two purposes:

• Add two numbers;

• Add two strings connect.

The specification does not define the behavior of the + operator on arrays, so JavaScript first converts the array into a string, and then performs the + operator on the string.

If you want to concatenate two arrays, you can use the concat method of the array:

[1, 2].concat([3, 4]) // [1, 2, 3, 4]

Overview of the + operator in javascript

JavaScript Has 6 built-in data types: (Annotation: From the link given, the original author should mean the data types of the original type system. JavaScript actually has two type systems. The first type system is using

<strong>typeof<span style="color: #ff0000;"></span> </strong> is called the primitive type system, and the second type system is based on it. The basis is developed from the type of object, that is, the object type system. The object type system is identified by instanceof )##.

#undefined
boolean
number
string
function

• ##object

• It should be noted that

null and []<strong> <span style="color: #ff0000;"> are two completely different types. When using </span></strong> typeof<strong> When <span style="color: #ff0000;"> is operated, they all return </span></strong> object. But when using the + operator, the way it works in these two cases is different. In JavaScript,

Array is not a basic type, its existence is just a sugar-coated syntax, it is actually Array Instance of class. (ps: function is actually the sugar-coated syntax for Function class instance.) If your mind is still clear now, it’s time to add some excitement. javascript

Object wrapper types such as

new Number(5), new Boolean(true ) and new String("abc")<strong> <span style="color: #ff0000;"> are also </span>object </strong> types, they are not Number, boolean, string. However, the arithmetic operators Number and Boolean represent numbers. Remember the + operator I mentioned earlier? Its operation objects are numbers and strings, that is,

Number, Boolean, String or number, boolean, string. The following table is the result type obtained after the + operator operates on different types

----------------------------------------------------------------------------------------
           | undefined | boolean | number | string | function | object | null   | array
----------------------------------------------------------------------------------------
undefined  | number    | number  | number | string | string   | string | number | string
boolean    | number    | number  | number | string | string   | string | number | string

number     | number    | number  | number | string | string   | string | number | string

string     | string    | string  | string | string | string   | string | string | string
function   | string    | string  | string | string | string   | string | string | string

object     | string    | string  | string | string | string   | string | string | string
null       | number    | number  | number | string | string   | string | number | string

array      | string    | string  | string | string | string   | string | string | string
-------------------------------------------------------------------------------------------

This table is applicable to Chrome 13, Firefox 6, Opera 11 and IE9. Homework: Check other browser compatibility.

Note: + operation on user-defined objects may not always produce a string result. This mainly depends on how the object type to native type conversion is implemented.

For example:

var o = { 
    valueOf : function () { return 4; } 
};

Calculating o + 2 will get 6, which is a number; calculating o + '2' will get '42', which is a string string.

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