The + Operator in JavaScript

The + operator is one of the most versatile — and most confusing — operators in JavaScript. It does two very different things depending on the types of its operands.

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1. Addition (Numbers)

If both operands are numbers, it behaves like normal math:

1 + 2    // 3
10 + 5   // 15
3.5 + 1.5 // 5

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2. String Concatenation

If any one operand is a string, + switches to string concatenation:

"1" + 2   // "12"
2 + "3"   // "23"
"Hello" + " World"  // "Hello World"

JavaScript prefers strings when one side is already a string.

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3. Type Coercion Magic (and Confusion)

This is where it gets tricky — JavaScript tries to convert types automatically before applying +.

true + 1        // 2        (true → 1)
false + 1       // 1        (false → 0)
null + 1        // 1        (null → 0)
undefined + 1   // NaN      (undefined → NaN)

How it works internally

JavaScript calls an internal ToPrimitive conversion before + runs:

Value	Converts To
true	1
false	0
null	0
undefined	NaN

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4. Objects & Arrays Behave Weirdly

Objects and arrays are converted to primitives (usually strings) before + is applied.

[1, 2] + [3, 4]   // "1,23,4"
[] + []            // ""
[] + {}            // "[object Object]"
{} + []            // 0  (in some contexts, {} is parsed as a block!)
[] + 1             // "1"
{} + 1             // 1

Why?

Arrays convert to strings using .toString():

[1, 2].toString()   // "1,2"
[3, 4].toString()   // "3,4"
// So: "1,2" + "3,4" = "1,23,4"

Objects convert to "[object Object]" by default:

({}).toString()   // "[object Object]"

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5. Order Matters (Left-to-Right Evaluation)

JavaScript evaluates + expressions left to right, and the type of the first result determines what happens next.

1 + 2 + "3"   // "33"
"1" + 2 + 3   // "123"

Why?

// Example 1:
1 + 2       // → 3       (number + number)
3 + "3"     // → "33"    (number + string = string)

// Example 2:
"1" + 2     // → "12"    (string + number = string)
"12" + 3    // → "123"   (string + number = string)

Tip: Parentheses can control evaluation order:

"1" + (2 + 3)   // "15"

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6. Unary + (Bonus)

There's also a single (unary) + that converts a value to a number:

+"123"     // 123
+"3.14"    // 3.14
+"abc"     // NaN
+true      // 1
+false     // 0
+null      // 0
+undefined // NaN
+""        // 0
+" "       // 0

This is a quick and common way to convert strings to numbers in JavaScript.

const input = "42"
const num = +input   // 42 (number)

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Summary Table

Expression	Result	Reason
1 + 2	3	Number + Number
"1" + 2	"12"	String operand → concatenation
true + 1	2	true coerces to 1
null + 1	1	null coerces to 0
undefined + 1	NaN	undefined coerces to NaN
[] + []	""	Both arrays convert to ""
[1,2] + [3,4]	"1,23,4"	Arrays stringify before concat
1 + 2 + "3"	"33"	Left-to-right: 3 then "33"
"1" + 2 + 3	"123"	Left-to-right: "12" then "123"
+"42"	42	Unary + converts to number
+"abc"	NaN	Can't convert to number

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Rule of thumb: When in doubt, explicitly convert types using Number(), String(), or parseInt() instead of relying on JavaScript's implicit coercion.