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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Decreased performance in code running in Temporal Dead Zone","articleBody":"\u003c!--\r\nThank you for reporting an issue.\r\n\r\nThis issue tracker is for bugs and issues found within Node.js core.\r\nIf you require more general support please file an issue on our help\r\nrepo. https://github.com/nodejs/help\r\n\r\n\r\nPlease fill in as much of the template below as you're able.\r\n\r\nVersion: output of `node -v`\r\nPlatform: output of `uname -a` (UNIX), or version and 32 or 64-bit (Windows)\r\nSubsystem: if known, please specify affected core module name\r\n\r\nIf possible, please provide code that demonstrates the problem, keeping it as\r\nsimple and free of external dependencies as you are able.\r\n-->\r\n\r\n* **7.9.0**:\r\n* **Linux mitsos-XPS-13-9360 4.10.0-19-generic #21~16.04.1-Ubuntu SMP Fri Apr 7 08:20:02 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux**:\r\n* **Subsystem**:\r\n\r\n\u003c!-- Enter your issue details below this comment. -->\r\n\r\n**I have created a detailed Stack Overflow question [here](http://stackoverflow.com/questions/43540849/javascript-temporal-dead-zone-nodejs-7-9-0-performance-decrease) so I will quote it here to get some answers since I do not know the internal mechanichs...\r\n\r\n**Quoted Question:**\r\n\r\nI have noticed in an other [question](http://stackoverflow.com/questions/37792934/why-is-let-slower-than-var-in-a-for-loop-in-nodejs) the performance difference in loops while using `let` and `var` variables declaration.\r\n\r\n\r\nThe initial  question is correctly answered that using `let` in the **for loop** is slower since `let` creates a new scope for each iteration to hold the value of `let` declared variable. More work to be done, so it is normal to be slower. Just as reference, I give the code and the results in NodeJS (7.9.0) execution time:\r\n\r\n*Please note that all javascript code below regards NodeJS version 7.9.0*\r\n\r\n**Regular Code:**\r\n\r\n    'use strict';\r\n    console.time('var');\r\n    for (var i = 0; i \u003c 100000000; i++) {}\r\n    console.timeEnd('var');\r\n    \r\n    \r\n    console.time('let');\r\n    for (let j = 0; j \u003c 100000000; j++) {}\r\n    console.timeEnd('let');\r\n\r\n**Output:**\r\n\r\n    var: 55.792ms\r\n    let: 247.123ms\r\n\r\nIn order to avoid the extra scope declaration for `j` in every iteration of the loop, we declare the `j` variable just before the loop. One would expect that this should now make the `let` loop performance match the one of `var`. **BUT NO!!!** This is the code and the result for reference:\r\n\r\n**Code with `let` defined before loop:**\r\n\r\n    'use strict';\r\n    console.time('var');\r\n    for (var i = 0; i \u003c 100000000; i++) {}\r\n    console.timeEnd('var');\r\n    \r\n    \r\n    console.time('let');\r\n    let j;\r\n    for (j = 0; j \u003c 100000000; j++) {}\r\n    console.timeEnd('let');\r\n\r\n**Output:**\r\n\r\n    var: 231.249ms\r\n\tlet: 233.485ms\r\n\r\nWe can see that, not only the `let` loop did not get any faster but also the `var` loop became as slow as the `let` one!!! The only explanation for this is that since we are not in any block or function, both variables are declared in the **global** module scope. However as referenced [here](http://exploringjs.com/es6/ch_variables.html#sec_temporal-dead-zone), the `let` declaration of the variable in the middle of the scope, creates a **temporal dead zone**, which leaves the variable `j` uninitialized, while the `var` initializes the variable as defined.\r\n\r\nSo running code in a temporal dead zone although the uninitialized variable is not referenced, must be quite slower....\r\n\r\nFinally to show the deference, we declare the `j` variable on top of the program to show the results of running it without the **temporal dead zone**.\r\n\r\n**Code without temporal dead zone:**\r\n\r\n    'use strict';\r\n    let j;\r\n    console.time('var');\r\n    for (var i = 0; i \u003c 100000000; i++) {}\r\n    console.timeEnd('var');\r\n    \r\n    \r\n    console.time('let');\r\n    for (j = 0; j \u003c 100000000; j++) {}\r\n    console.timeEnd('let');\r\n\r\n**Output:**\r\n\r\n\tvar: 55.586ms\r\n\tlet: 55.009ms\r\n\r\n**Now both `let` and `var` loops have similar optimized performance!**\r\n\r\nDoes anyone know whether my assumption about temporal dead zone performance is correct, or provide a different explanation???\r\n\r\n","author":{"url":"https://github.com/mitsos1os","@type":"Person","name":"mitsos1os"},"datePublished":"2017-04-21T11:16:09Z","url":"https://github.com/nodejs/node/issues/12568"}