Ecma is a standard for making a scripting language many scripting languages are using this standard such as actionscript, jscript, rhino, v8, Nashorn, Carakan, and indeed javascript.
It is not however a comprehensive javascript reference. It will not teach you how to use cookies or how to generate a pdf or how to save to excel etc. It also does not teach you basic javascript like keyboard key assignment to functions etc. It does however contain explanations of how basic mathematical functions are defined in javascript etc. This information would be useful if you were going to write javascript but useless to somebody trying to learn javascript.
This is an ecma script example