Because JavaScript has semantic types like List, Date, Number, String, Boolean, witch can easily be figured out by their context, like age is a number, name is

Because JavaScript has semantic types like List, Date, Number, String, Boolean, witch can easily be figured out by their context, like age is a number, name is a a string, married is a boolean, children is a list, born is a date, and adding type declarations are unnecessary, witch would only add extra cognitive load to the programmer. It's possible to be an productive JavaScript programmer without even knowing about types and how they are implemented. If the programmer however makes a mistake, like concatenating name and age, it will be caught during runtime, manually testing, automatic testing, runtime testing, or real time static analysis, or all of these five. Being able to concatenate and mix types like number and string is also a language feature that avoids a lot of boilerplate code.