Passing Typescript "Map" as a part of Angular4 HTTP POST request

I have an Angular 4 Project where I am creating a sample Map as follows:

let sampleMap = new Map<string, string>();
sampleMap.set('key1','value1');


Now I am passing this Map as a parameter to a Angular 4 Post method which connects to a Spring Boot Rest backend as follows

Angular 4 Code:

this.http.post('http://localhost:8080/data/posturl', sampleMap).map((response: Response) => {
return <string>response.text();
})


Spring Boot Rest Backend Code:

@RequestMapping("/posturl")
public String launch(@RequestBody Map<String, String> sampleMap) {
System.out.println("Received=" + sampleMap);
return "SUCCESS";
}


Although when I try to print the 'sampleMap' as shown above, it print a blank map like follows:

Received={}


I am using Typescript version '~2.3.3' and my 'tsconfig.json' mentions target as 'es5'. Can someone please explain this behavior?

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