@babel/plugin-transform-block-scoping

    In

    Out

    1.   var _a = 3;
    2. }
    4. var a = 3;

    This plugin also validates all const variables. Reassignment of constants is a runtime error and it will insert the necessary error code for those.

    1. npm install --save-dev @babel/plugin-transform-block-scoping

    With options:

    1. {
    3.     [
    4.       "@babel/plugin-transform-block-scoping",
    5.       {
    6.       }
    7.     ]
    8.   ]
    9. }
    1. babel --plugins @babel/plugin-transform-block-scoping script.js

    , defaults to false.

    In cases such as the following it’s impossible to rewrite let/const without adding an additional function and closure while transforming:

    1. for (let i = 0; i < 5; i++) {
    2.   setTimeout(() => console.log(i), 1);
    3. }

    boolean, defaults to false.

    By default this plugin will ignore the temporal dead zone (TDZ) for block-scoped variables. The following code will not throw an error when transpiled with Babel, which is not spec compliant:

    1. i;
    2. let i;

    If you need these errors you can tell Babel to try and find them by setting for this plugin. However, the current implementation might not get all edge cases right and its best to just avoid code like this in the first place.