The use of let and const is done as an alternative to the var function during the declaration of variables. Unlike var , let and const are block scoped. Another difference that let and const have when compared to var is the fact that while var gives a undefined error when accessed before declaring, let and const give a ReferenceError if they are accessed before declaration.
The error is due to TDZ, or Temporal Dead Zone. TDZ starts at the beginning of the block where let is declared and ends when let is encountered again in the user code.
TDZ is very useful as it helps to avoid bugs which appear when a variable is changed before its declaration is reached.
While let and const are similar, they are not exactly the same. The main difference between let and const is that const variables need to be declared using an initializer, or it will generate an error.
Also declaration of the variable with the same name will throw an error. This helps to fix mistakes where one loses a reference when a variable is reassigned.
Let and Const are already replacing Var in many codebases, and are additional features of ES6 that will improve code.
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