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What is the difference between "let" and "var"?

ECMAScript 6 introduced the let statement.

I've heard that it's described as a local variable, but I'm still not quite sure how it behaves differently than the var keyword.

What are the differences? When should let be used instead of var?

Answer*

### What's the difference between `let` and `var`? 

- A variable defined using a `var` statement is known throughout [**the function**][1] it is defined in, from the start of the function. *(\*)*
- A variable defined using a `let` statement is only known in [**the block**][2] it is defined in, from the moment it is defined onward. *(\*\*)*

To understand the difference, consider the following code:

    // i IS NOT known here
    // j IS NOT known here
    // k IS known here, but undefined
    // l IS NOT known here

    function loop(arr) {
        // i IS known here, but undefined
        // j IS NOT known here
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here

        for( var i = 0; i < arr.length; i++ ) {
            // i IS known here, and has a value
            // j IS NOT known here
            // k IS known here, but has a value only the second time loop is called
            // l IS NOT known here
        };

        // i IS known here, and has a value
        // j IS NOT known here
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here

        for( let j = 0; j < arr.length; j++ ) {
            // i IS known here, and has a value
            // j IS known here, and has a value
            // k IS known here, but has a value only the second time loop is called
            // l IS NOT known here
        };

        // i IS known here, and has a value
        // j IS NOT known here
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here
    }

    loop([1,2,3,4]);

    for( var k = 0; k < arr.length; k++ ) {
        // i IS NOT known here
        // j IS NOT known here
        // k IS known here, and has a value
        // l IS NOT known here
    };

    for( let l = 0; l < arr.length; l++ ) {
        // i IS NOT known here
        // j IS NOT known here
        // k IS known here, and has a value
        // l IS known here, and has a value
    };

    loop([1,2,3,4]);

    // i IS NOT known here
    // j IS NOT known here
    // k IS known here, and has a value
    // l IS NOT known here

Here, we can see that our variable `j` is only known in the first for loop, but not before and after. Yet, our variable `i` is known in the entire function.

Also, consider that block scoped variables are not known before they are declared because they are not hoisted. You're also not allowed to redeclare the same block scoped variable within the same block. This makes block scoped variables less error prone than globally or functionally scoped variables, which are hoisted and which do not produce any errors in case of multiple declarations.

------

### Is it safe to use `let` today?

Some people would argue that in the future we'll ONLY use let statements and that var statements will become obsolete. JavaScript guru [**Kyle Simpson**][3] wrote [**a very elaborate article on why he believes that won't be the case**][4].

Today, however, that is definitely not the case. In fact, we need actually to ask ourselves whether it's safe to use the `let` statement. The answer to that question depends on your environment:

- If you're writing server-side JavaScript code ([**Node.js**][5]), you can safely use the `let` statement.

- If you're writing client-side JavaScript code and use a browser based transpiler (like [**Traceur**][6] or [**babel-standalone**][10]), you can safely use the `let` statement, however your code is likely to be anything but optimal with respect to performance.

- If you're writing client-side JavaScript code and use a Node based transpiler (like the [**traceur shell script**][11] or [**Babel**][12]), you can safely use the `let` statement. And, because your browser will only know about the transpiled code, performance drawbacks should be limited.

- If you're writing client-side JavaScript code and don't use a transpiler, you need to consider browser support.

    There are still some browsers that don't support `let` at all :
    
[![enter image description here][7]][7]

------

### How to keep track of browser support

For an up-to-date overview of which browsers support the `let` statement at the time of your reading this answer, see [**this `Can I Use` page**][8].

------

*(\*) Globally and functionally scoped variables can be initialized and used before they are declared because JavaScript variables are [**hoisted**][9].* This means that declarations are always moved to the top of the scope.

*(\*\*) Block scoped variables are not hoisted*


  [1]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/function
  [2]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/block
  [3]: https://twitter.com/getify
  [4]: https://davidwalsh.name/for-and-against-let
  [5]: https://nodejs.org/en/
  [6]: https://github.com/google/traceur-compiler
  [7]: https://i.sstatic.net/J9kEC.png
  [8]: http://caniuse.com/#search=let
  [9]: https://developer.mozilla.org/en-US/docs/Glossary/Hoisting
  [10]: https://github.com/babel/babel-standalone
  [11]: https://github.com/google/traceur-compiler/wiki/Compiling-Offline
  [12]: https://babeljs.io/

Edit Summary*

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23

regarding answer v4: i IS known everywhere in the function-block! It starts as undefined (due to hoisting) until you assign a value! ps: let is also hoisted (to the top of it's containing block), but will give a ReferenceError when referenced in the block before first assignment. (ps2: I'm a pro-semicolon kinda guy but you really don't need a semicolon after a block ). That being said, thanks for adding the reality-check regarding support!
– GitaarLAB
Commented May 21, 2016 at 4:41
@GitaarLAB : According to the Mozilla Developer Network : "In ECMAScript 2015, let bindings are not subject to Variable Hoisting, which means that let declarations do not move to the top of the current execution context." - Anyway, I made a few improvements to my answer that should clarify the difference in hoisting behavior between let and var!
– John Slegers
Commented Feb 26, 2018 at 23:37
2

Your answer improved a lot (I thoroughly checked). Note that same link you referenced in your comment also says: "The (let) variable is in a "temporal dead zone" from the start of the block until the initialization is processed." That means that the 'identifier' (the text-string 'reserved' to point to 'something') is already reserved in the relevant scope, otherwise it would become part of the root/host/window scope. To me personally, 'hoisting' means nothing more than reserving/linking declared 'identifiers' to their relevant scope; excluding their initialization/assignment/modifyability!
– GitaarLAB
Commented Mar 1, 2018 at 18:16
And..+1. That Kyle Simpson article you linked is an excellent read, thank you for that! It is also clear about the "temporal dead zone" aka "TDZ". One interesting thing I'd like to add: I've read on MDN that let and const were recommended to only use when you actually need their additional functionality, because enforcing/checking these extra features (like write-only const) result in 'more work' (and additional scope-nodes in the scope-tree) for the (current)engine(s) to enforce/check/verify/setup.
– GitaarLAB
Commented Mar 1, 2018 at 18:17
2

Note that MDN says that IE DOES interpret let correctly. Which is it? developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
– Katinka Hesselink
Commented Feb 6, 2019 at 12:42

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What is the difference between "let" and "var"?

Answer*