Numbers, Strings and Coercion

Terminology

Coercion: Implicitly converting a value from one type to another.
Arithmetic operator: Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value.
Addition Operator (+): The addition operator produces the sum of numeric operands OR string concatenation.
String Concatenation: Joining two or more strings together.

Untyped languages and coercion

It's important to know that Javascript is an untyped language, meaning that a variable will often be converted from one data type (such as a number) to another (a string). This often "helps" programmers type less code but will sometimes cause problems when you don't expect it.

See: JavaScript type coercion explained for more information

Key Takeaways

The addition + does two jobs:
1. It adds numbers together;
2. It concatenates (joins) two strings together.
The + operator will concatenate values if one or more is a string.
- For example: 1 + 1 is 2, but 1 + "1" is "11".
When a number is needed but the type cannot be converted to a number, you'll see the dreaded NaN property.
When you need to concatenate a string with a variable, it's recommended you use the new (ES6) template literal syntax.