Adding Numbers and Strings

JavaScript uses the + operator for both addition and concatenation.

Numbers are added. Strings are concatenated.

If you add two numbers, the result will be a number:

var x = 10;
var y = 20;
var z = x + y; // z will be 30 (a number)

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If you add two strings, the result will be a string concatenation:

var x = "10";
var y = "20";
var z = x + y; // z will be 1020 (a string)

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If you add a number and a string, the result will be a string concatenation:

var x = 10;
var y = "20";
var z = x + y; // z will be 1020 (a string)

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If you add a string and a number, the result will be a string concatenation:

var x = "10";
var y = 20;
var z = x + y; // z will be 1020 (a string)

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A common mistake is to expect this result to be 30:

var x = 10;
var y = 20;
var z = "The result is: " + x + y;

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A common mistake is to expect this result to be 102030:

var x = 10;
var y = 20;
var z = "30";
var result = x + y + z;

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The JavaScript interpreter works from left to right.

First 10 + 20 is added because x and y are both numbers.

Then 30 + "30" is concatenated because z is a string.

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