Typing: Static vs Dynamic, Strong vs Weak

Static vs Dynamic

A statically typed language requires you to declare the type explicitly. Such type markers are evaluated at compile time.

E.g., in Java:

int num = 3;
num = 'a'; // Type error is thrown at compile time

Obviously JavaScript doesn't exhibit this behavior. The type of a given variable can change at runtime. JavaScript is dynamically typed.

Note that in JavaScript's case, when we say "type of a variable", this is a bit misleading because we are actually talking about the type of the value that the variable points to. The variable itself can point at, and be re-pointed at, any value. The values themselves are what are tagged as being a Number or a String, etc.

Strong vs Weak

A strongly typed language will require you to do something sensible with your values.

This won't fly in Python:

>>> x = 'a'
>>> x + 3
TypeError: cannot concatenate 'str' and 'int' objects

But in the same situation, JavaScript will happily try to do something it considers reasonable:

> x='a'
"a"

> x+3
"a3"

This tolerant stance when evaluating mixed types makes JavaScript a weakly typed language.