Why Does `[1,2] [3,4]` Result in `\'1,23,4\'` in JavaScript?

JavaScript Concatenation: Why [1,2] [3,4] = "1,23,4"?

When adding arrays in JavaScript using the operator, you may encounter an unexpected result. For instance, the operation [1,2] [3,4] will produce "1,23,4" instead of the expected [1,2,3,4].

This occurs because the operator is not defined for arrays. Instead, JavaScript attempts to convert the arrays to strings and concatenate them. In this case, each array is converted to a comma-separated string, resulting in "1,2" and "3,4." These strings are then concatenated to produce "1,23,4."

A Comprehensive Overview of Operator Behavior

To further clarify this behavior, let's examine how the operator works with different data types:

Operand 1	Operand 2	Result Type
Undefined	Any	String
Null	Any	String
Boolean	Any	String
Number	Number	Number
String	String	String
Object	Anything	String

Note that in Chrome13, FF6, Opera11, and IE9, objects generally produce a string result when used with the operator. However, for objects such as Number and Boolean, the behavior may vary depending on their implementation of object to primitive conversion.

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