So strict mode in javascript is basically used in javascript file to avoid certain mistakes while writing code, we can use strict mode directly on js file and on inside the functions too but we cant use on predefined functions 
for example if we use struct mode and assigned value to value to variable without declaring it , it will throw an error example x = 5 
Another example is when we assign number to NaN , it will throw an error too in strict mode but in sloppy mode it will just silently mode 
sloppy mode is basically opposite of strict mode

So strict mode in javascript is basically used in javascript file to avoid certain mistakes while writing code, we can use strict mode directly on js file and on inside the functions too but we cant use on predefined functions  
  
for example if we use struct mode and assigned value to value to variable without declaring it , it will throw an error  
example  
x = 5  
  
Another example is when we assign number to NaN , it will throw an error too in strict mode but in sloppy mode it will just silently mode  
  
sloppy mode is basically opposite of strict mode

What is strict mode in javascript?