The let keyword can be used in four ways :
1. As a variable declaration like var;
2. In a for or for/in loop, as a substitute for var;
3. As a block statement, to define new variables and explicitly delimit their scope; and
4. To define variables that are scoped to a single expression.
Slow for PhantomJS is a command line script that allows page performance analysis from live URLs, unlike YSlow for Command Line (HAR) where a pre-generated HAR file is needed in order to analyze page performance.