Declaring javascript variables as specific types [closed]

问题

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.

Closed 6 years ago.

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?

回答1:

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.

来源：https://stackoverflow.com/questions/14202743/declaring-javascript-variables-as-specific-types