JavaScript is a object-based scripting language and it is light weighted. It is first implemented by Netscape (with help from Sun Microsystems). JavaScript was created by Brendan Eich at Netscape in 1995 for the purpose of allowing code in web-pages (performing logical operation on client side).
Using HTML we can only design a web page but you can not run any logic on web browser like addition of two numbers, check any condition, looping statements(for, while), decision making statement(if-else) etc. All these are not possible using HTML so for perform all these task we use JavaScript.
Using HTML we can only design a web page if we want to run any programs like c programming we use JavaScript. Suppose we want to print sum of two number then we use JavaScript for coding.