6a7223af29
`this.blockPath.get("body")` constructs an array of paths corresponding to each node in `blocks.body` so takes O(n) time if n is that length. We were re-constructing that array on each iteration, so the entire loop was O(n^2).
On files with many statements in a single block (such as Rollup-generated bundles), this takes a large portion of time. In particular, this makes transforming react-dom.development.js about 40% faster. Not that you should be transforming our bundle with Babel.
Test Plan:
Make an HTML file with these three lines and watch it in the Chrome Performance tab to see timings (on my machine: 2.9s before, 1.6s after):
```
<!DOCTYPE html>
<script src="https://unpkg.com/babel-standalone@7.0.0-beta.3/babel.js"></script>
<script type="text/babel" src="https://unpkg.com/react-dom@16.2.0/umd/react-dom.development.js"></script>
```
@babel/plugin-transform-block-scoping
Compile ES2015 block scoping (const and let) to ES5
Examples
In
{
let a = 3
}
let a = 3
Out
{
var _a = 3;
}
var a = 3;
Installation
npm install --save-dev @babel/plugin-transform-block-scoping
Usage
Via .babelrc (Recommended)
.babelrc
Without options:
{
"plugins": ["@babel/plugin-transform-block-scoping"]
}
With options:
{
"plugins": [
["@babel/plugin-transform-block-scoping", {
"throwIfClosureRequired": true
}]
]
}
Via CLI
babel --plugins @babel/plugin-transform-block-scoping script.js
Via Node API
require("@babel/core").transform("code", {
plugins: ["@babel/plugin-transform-block-scoping"]
});
Options throwIfClosureRequired
In cases such as the following it's impossible to rewrite let/const without adding an additional function and closure while transforming:
for (let i = 0; i < 5; i++) {
setTimeout(() => console.log(i), 1);
}
In extremely performance-sensitive code, this can be undesirable. If "throwIfClosureRequired": true is set, Babel throws when transforming these patterns instead of automatically adding an additional function.