There are some quick notes of "use strict" in javascript:

"use strict":
– Is a string
– Define in the top of .js file or top of function
– If old browsers don’t support it, they will omit it

How to use?

• Prevent assigning value to undefined variable

=> Comment the first line, run the code again & see there is no error

• Prevent using let as a variable name
  • let  is a keyword  that used for block scope from ES 6
• Don’t allow to delete variables, functions or arguments

• Safer to use eval() function
  • eval() evaluates a javascript expression by providing a string  i.e. eval("var a = 2") <=> var a = 2;
  •  KEEP variables which are defined in eval function are only in eval’s scope