Why does JavaScript return \"1,23,4\" when adding arrays with the operator?

Why Does JavaScript Return "1,23,4" When Adding Array [1,2] to [3,4]?

When attempting to add elements of two arrays using the operator, unexpected results may arise, as illustrated by the following code snippet:

[1,2] + [3,4]

This expression returns "1,23,4" rather than "[1,2,3,4]".

Understanding the Issue

The operator is not specifically defined for arrays in JavaScript. Instead, when adding arrays, Javascript performs the following steps:

1. Converts both arrays into strings.
2. Concatenates the resulting strings, resulting in a comma-separated list of elements.

Fixing the Behavior

To correctly add the elements of two arrays, use the spread operator instead of the operator:

[1,2, ...[3,4]] // [1,2,3,4]

Overview of Operator Behavior

The operator's behavior varies depending on the operand types involved:

Operand Types	Result Type
undefined, null, boolean	Number
number	Number
string	String
object	String (except for certain objects like Number and Boolean, where the result may vary depending on the implementation)

Refer to the provided table or the referenced jsfiddle for a complete overview of result types across various operand combinations.

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