Declaring javascript variables as specific types

javascript variables variable-declaration
CBredlow picture CBredlow · Jan 7, 2013 · Viewed 50k times · Source

The title says it all, but I will provide more clarification:

After seeing many samples of javascript where all variables are declared as type var, and seeing support for other datatypes, why aren't variables of a specific datatype declared as such? Meaning, why isn't this:

string hello = 'Hello, World'

used instead of

var hello = 'Hello, World'

Looking at sites like OReilly Javascript shows that there are reserved words for other types. Again, why aren't they used? Wouldn't it make lines like this: typeof(variable)==='string'; no longer needed?

Answer

Matt Ball picture Matt Ball · Jan 7, 2013

Quite simply, JavaScript variables do not have types. The values have types.

The language permits us to write code like this:

var foo = 42;
foo = 'the answer';
foo = function () {};

So it would be pointless to specify the type in a variable declaration, because the type is dictated by the variable's value. This fairly common in "dynamic" languages.

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