具有开关盒的函数中“ let”和“ var”之间的区别

Question

i was working on a functionality, i have found an issue while using let inside the switch case of a function. if i change to var it works. I have read difference between the let and var. But is this case is causing issue of block level scope. Little bit confused.

Any help appreciated

 var a = "sample data" const returnDataOfLet = (a) => { switch(typeof a){ case "string": let a = a.split(" ") // will throw an error change let to var it will work return a } } console.log(returnDataOfLet(a)) 

Answer 1

Inside the block you declare let a . This creates a new variable a .

Then you call a.split . This throws an error because a is undefined (it has to be undefined : you haven't give it a value yet).

The program stops at the point because of the exception.

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If you use var instead of let , then the var is ignored completely because there is already an a declared in that scope.

Since you haven't created a new variable, a is the argument you passed to the function.

This is a string, so you can call split on it and assign the result back to the a variable declared in the argument list.

Answer 2

Here is a elaborate guide on this topic. Generally:

• let is block scoped, while var is function scoped (not visible from outside the function).
• let cannot be accessed before declaration (var can be, and is actually the source of many bugs and confusion in the JS world)
• let cannot be redeclared

The way you use let here, the variable is redeclared at any iteration, that causes the error. You can always reassign the value of a though.

 let a = "sample data"; const returnDataOfLet = (a) => { switch (typeof a) { case "string": return a.split(" "); } } console.log(returnDataOfLet(a)); 

Answer 3

The globally defined var a is not the same a which is passed to the function as parameter . Its just like an alias for a variable . and also in case " string " no need to again create a as it is already there in the function. Here it is not a problem with scopes but definition and parameters .

 var a = "sample data" const returnDataOfLet = (a) => { switch(typeof a){ case "string": a = a.split(" ") // will throw an error change let to var it will work return a } } console.log(returnDataOfLet(a)) 

If you want to use let use another variable name

 var a = "sample data" const returnDataOfLet = (a) => { switch(typeof a){ case "string": let b = a.split(" ") // will throw an error change let to var it will work return b } } console.log(returnDataOfLet(a))