14d3c2e256
When you write
```
for (const x of l) {
setTimeout(() => x);
}
```
we need to add a closure because the variable is meant to be block-scoped and recreated each time the block runs. We do this.
However, we also add the closure when no loop is present. This isn't necessary, because if no loop is present then each piece of code runs at most once. I changed the transform to only add a closure if a variable is referenced from within a loop.
babel-plugin-transform-es2015-block-scoping
Compile ES2015 block scoping (const and let) to ES5
Installation
npm install --save-dev babel-plugin-transform-es2015-block-scoping
Usage
Via .babelrc (Recommended)
.babelrc
Without options:
{
"plugins": ["transform-es2015-block-scoping"]
}
With options:
{
"plugins": [
["transform-es2015-block-scoping", {
"throwIfClosureRequired": true
}]
]
}
Via CLI
babel --plugins transform-es2015-block-scoping script.js
Via Node API
require("babel-core").transform("code", {
plugins: ["transform-es2015-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.