Question

var params = {a:1,b:2};
var str = '<a href="#" onclick="doSomething('+params+')">aaaa</a>';
document.write(str);

when I click the <a> on the page,it throws “Uncaught SyntaxError: Unexpected identifier”.I can’t understand.

Answer 1

The reason is that when you use string concatenation, params is casted to string, as result you get something like [object Object] in parenthesis.

You should better put params as var params = '{a:1,b:2}';.

Upd.
As suggested in comments, another viable approach is using JSON.stringify:

var params = {a:1,b:2};
var str = '<a href="#" onclick="doSomething('
    + JSON.stringify(params)
    + ')">aaaa</a>';
document.write(str);

Please, pay attention that JSON.stringify may not be supported by older browsers and you’ll need to include additional libraries to make them work.