var a:number = 12345;
var b:string = <string> a;
// laygroundSingle.ts(24,18): error TS2352: Neither type 'number' nor type 'string' is assignable to the other.

var a:number = 12345;
var b:string = <string><any> a;
console.log(typeof b)
// "number" PlaygroundSingle.js:19:1

var b:string = String(a);
// or
var b:string = a.toString();

var a:number = 12345;
// 使用 new 的时候类型必须是 String 而非 string ，否则无法编译通过
var b:String = new String(a);
// 不使用 new 则无所谓
var c:string = String(a);
console.log(a);
console.log('--------b');
console.log(typeof b);
console.log(b);
console.log(b.length);
console.log('--------c');
console.log(typeof c);
console.log(c);
console.log(c.length);

12345 PlaygroundSingle.js:22:9
“——–b” PlaygroundSingle.js:23:9
“object” PlaygroundSingle.js:24:1
String [ “1”, “2”, “3”, “4”, “5” ] PlaygroundSingle.js:25:9
5 PlaygroundSingle.js:26:9
“——–c” PlaygroundSingle.js:27:9
“string” PlaygroundSingle.js:28:1
“12345” PlaygroundSingle.js:29:9
5