SubString

String.substring ( start , end )

One argument, we get the string starting from the specified index Inclusive until the end of the string.

sText = "abcdef" 
sSubText = sText.substring(2); 
console.log(sSubText);    // ab 

Two arguments, we get the string starting from the first number to the end number Exclusive.

sText = "abcdef" 
sSubText = sText.substring(0, 2); 
console.log(sSubText);    // ab 

String.substr ( start , length )

sText.substr(0,3) 

The second argument is the length