startsWith not works in javascript for me..

I am using startsWith function in javascript., its working in one page but not in another page.
var str="Hello World";
if(str.startsWith('Hello'))
{   // This is working in a pae.
  }
but the same thing i have used in another page but there it is not working giving me error like,
Object Hello World has no method 'startsWith'
Both pages are in a same project and same folder too... then why this happening for that page only...?
...