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function prop(obj, key) {
  return obj[key]
}

function prop(obj:{}, key: string) {
  return obj[key] 
}

 const todo = {
      id : 1,
      text : 'sth',
      due : new Date(2019,12,22)
 }
 const id = prop(todo,'id') // any
 const text = prop(todo,'text') //any 
 const due = prop(todo,'due') // any
 Without further information, TypeScript can not know which value will be passed for 
 the key parameter, so it can not infer a more specific return type for the prop
 function. 

 function prop<T, K extends keyof T>(obj : T, key: K){
     return obj[key]
 }
 const id = prop(todo,'id') // number
 const text = prop(todo,'text') // string
 const due = prop(todo,'due') // Date

React Effect

function Counter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    const id = setInterval(() => {
      setCount(c => c + 1);
    }, 1000);
    // Tips effect 可选的清除机制。每个effct 都可以返回一个清除函数。
    return () => clearInterval(id);
  }, []); // only re-run the effect if count changes
  return <span>{count}</span>;
}

var MyModules = (function Manager() {
	var modules = {};

	function define(name, deps, impl) {
		for (var i=0; i<deps.length; i++) {
			deps[i] = modules[deps[i]];
		}
		modules[name] = impl.apply( impl, deps );
	}

	function get(name) {
		return modules[name];
	}

	return {
		define: define,
		get: get
	};
})();

MyModules.define( "bar", [], function(){
	function hello(who) {
		return "Let me introduce: " + who;
	}

	return {
		hello: hello
	};
} );


MyModules.define("foo",["bar"],function(bar){
    var hungry = "hippo";
    function awesome() {
        console.log(bar.hello(hungry).toUpperCase())
    }
    return {
        awesome : awesome
    }
})

var foo = MyModules.get( "foo" );
foo.awesome()