What Happens When You Use the + Operator Between Two Arrays in JavaScript? – Asked in an IBM Interview

Understanding the Code Step by Step

The given JavaScript snippet involves array addition using the + operator, which is an interesting case because arrays in JavaScript are objects, and the + operator does not perform numerical addition when used with objects. Instead, it performs type coercion.

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Step 1: Understanding the Given Code

const x = [1];  // Array with one element: [1]
const y = [2];  // Another array with one element: [2]
console.log(x + y);

Here:

• x is an array containing a single element [1].
• y is an array containing a single element [2].

The key operation is:

x + y

Since both x and y are arrays, JavaScript must determine how to handle addition (+) between two objects.

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Step 2: How Does JavaScript Handle x + y?

🔹 Rule: When using + with objects, JavaScript converts them to primitive values.

JavaScript follows these steps:

1. Convert arrays to strings using .toString()
2. Perform string concatenation (+) instead of numeric addition

Step 2.1: Array to String Conversion

Whenever JavaScript needs to convert an array to a primitive, it calls the .toString() method of the array.

👉 Calling .toString() on x and y

console.log([1].toString());  // "1"
console.log([2].toString());  // "2"

Step 2.2: Concatenating the Strings

Now the expression:

x + y

is evaluated as:

"1" + "2"

Since one operand is a string, JavaScript treats the + operator as string concatenation, producing:

"12"

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Step 3: Logging the Output

console.log(x + y);

Console Output

"12"

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Remember key points:

1. Arrays are objects in JavaScript, and the + operator does not perform numeric addition on arrays.
2. JavaScript first converts arrays to strings using .toString(), which joins elements with commas (,) by default.
1. Example : [1,2] → "1,2"
3. Since the resulting values are strings, + performs string concatenation, not arithmetic addition.

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✔ Final Answer:

D) 12

This question appeared in the IBM interview, testing the candidate’s knowledge of type coercion and the concatenation (+) operator in JavaScript.