What is the difference between var and let in Typescript?

Question

I submitted a question on stack overflow asking how I could stop the putTestQuestionResponses() function from executing IF a previous version was already executing.

The reply was to add in a processing flag which is here on line 2 of this code.

Can you tell me why use a "let" instead of a "var" here?

var promisePutTestQuestion;
let processing = false;

onEnter: ['$interval', 'questionService',
         ($interval, qus: IQuestionService) => {
  promisePutTestQuestion = $interval(() => {
     if (processing)
         return;
     processing = true;
     qus.putTestQuestionResponses()
     .then(() => processing = false)
  }, 5 * 1000);
}],
onExit: ['$interval', ($interval) => {
        $interval.cancel(promisePutTestQuestion);
}]

Answer 1

var declaration is function scoped and let declaration is block scoped.

See https://basarat.gitbooks.io/typescript/content/docs/let.html for more details.

Answer 2

example:

// demo: var
for(var i =0; i<5; i++){
   console.log(i) 
}//finally i =5
console.log(i) // i=5

// demo: let 
for(let i = 0; i<5; i++){
   console.log(i)
}
console.log(i)// i is undefined

Answer 3

var variables in JavaScript are function scoped. This is different from many other languages (C#, Java, etc.) where the variables are block scoped. If you bring a block scoped mindset to JavaScript, you would expect the following to print 123, instead it will print 456:

var foo = 123;
if (true) {
    var foo = 456;
}

console.log(foo); // 456

This is because { does not create a new variable scope. The variable foo is the same inside the if block as it is outside the if block. This is a common source of errors in JavaScript programming. This is why TypeScript (and ES6) introduces the let keyword to allow you to define variables with true block scope. That is, if you use let instead of var, you get a true unique element disconnected from what you might have defined outside the scope. The same example is demonstrated with let:

let foo = 123;
if (true) {
    let foo = 456;

}

console.log(foo); // 123

Answer 4

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
}

I found this here

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let

Answer 5

It's all about accessibility. If you use let then it will be accessible under that scope only not outside of the function, if, else scope. but var we can access outside of the for, if, else.

See below code

public selectedLocation(country)
  {  
     if(instance==this.list[0])
    {
      var obj=this.productArray
    }
    
    for(let i = 0; i < this.obj.length; i++)
    {
       obj=this.productPending
    }
  }

Above code is working with var obj but this will not work with let obj for the for loop.