Differencebetween 'Let' and 'Const' Ecmascript 2015 (Es6)

What is the difference between 'let' and 'const' ECMAScript 2015 (ES6)?

What you're seeing is just an implementation mistake. According to the ES6 spec wiki on const, const is:

A initialize-once, read-only thereafter binding form is useful and has
precedent in existing implementations, in the form of const
declarations.

It's meant to be read-only, just like it currently is. The ES6 implementation of const in Traceur and Continuum are buggy (they probably just overlooked it)

Here's a Github issue regarding Traceur not implementing const

What is the core difference between using of (let) and (const) when we declare of a variable in javascript ES6 (ES2015)?

The similarity between let and const is that they both have block scope. It means they are only available in the block they are declared.

Difference between them is that variables declared with let can be assigned a new value but variables declared with const cannot be assigned a new value.

let x = 3;x = 'changed';
const y = 33;y = 'changed' //throws error.

What is the difference between let and var?

Scoping rules

The main difference is scoping rules. Variables declared by var keyword are scoped to the immediate function body (hence the function scope) while let variables are scoped to the immediate enclosing block denoted by { } (hence the block scope).

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar); // Foo Bar

  {
    var moo = "Mooo"
    let baz = "Bazz";
    console.log(moo, baz); // Mooo Bazz
  }

  console.log(moo); // Mooo
  console.log(baz); // ReferenceError
}

run();

for...of loop. Should I use const or let?

Why use const and when to use let in for...of loops?

If there are no assignments to the identifier within the loop body, it's a matter of style whether you use let or const.

Use const if you want the identifier within the loop body to be read-only (so that, for instance, if someone modifies the code later to add an assignment, it's a proactive error). Use let if you want to be able to assign to it (because you have an assignment in your code, or you want someone to be able to add one later without changing the declaration).

You can do this with for-of and for-in loops. A for loop's control variable is normally not constant (since in the normal case you update it in the "update" clause of the for; if you don't, for may be the wrong loop to use), so you normally use let with it.

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For clarity, here's an example with an assignment within the loop body:

for (let str of ["a", " b", " c "]) {
    str = str.trim();
//  ^^^^^----- assignment to the identifier
    console.log(`[${str}]`);
}

const vs let when calling require

const can be normally used when you don't want your program

1. to assign anything to the variable

   "use strict";
   const a = 1;
   a = 2;
   

   will produce TypeError: Assignment to constant variable..

2. to use the variable without explicitly initializing.

   "use strict";
   const a;
   

   will produce SyntaxError: Unexpected token ;

Simply put, I would say,

• use const whenever you want some variables not to be modified

• use let if you want the exact opposite of const

• use var, if you want to be compatible with ES5 implementations or if you want module/function level scope.

Use let only when you need block level scoping, otherwise using let or var would not make any difference.

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