const
                                            value1 =
                                            "5"
                                            ;
                                            const
                                            value2 =
                                            9
                                            ;
                                            let
                                            sum =
                                            value1 +
                                            value2;
                                            console.
                                            log
                                            (
                                            sum)
                                            ;
                                        
                                    

In the above example, JavaScript has coerced the 9 from a number into a string and then concatenated the two values together, resulting in a string of 59 . JavaScript had a choice between a string or a number and decided to use a string.

The compiler could have coerced the 5 into a number and returned a sum of 14 , but it did not. To return this result, you'd have to explicitly convert the 5 to a number using the Number() method:

                                        
                                            sum =
                                            Number
                                            (
                                            value1)
                                            +
                                            value2;
                                        
                                    

• Type conversion (Wikipedia)
• Glossary
  • Type
  • Type conversion