为什么分配给const变量不会导致错误？

Question

Demo: http://jsfiddle.net/rmrG9/

Code:

const a = 10
a = 20
alert('a: ' + a)

The output is:

a: 10

Can you explain why the statement a = 20 did not throw an error?

Answer 1

Quoting the doc ( MDN ):

None of the major browsers produce any notices or errors if you assign another value to a constant. The return value of such an operation is that of the new value assigned, but the reassignment is unsuccessful only in Firefox and Chrome (at least since version 20).

As const is really just a proposal at the moment, browser vendors are free to implement the way they see fit. Note that the standard will most probably define const to be block-scoped (and currently it's function-scoped, just as var and function ).