@babel/plugin-transform-block-scoping
Examples
In
{
let a = 3
}
let a = 3
Out
{
var _a = 3;
}
var a = 3;
Constant checks
This plugin also validates all const
variables.
Reassignment of constants is a runtime error and it will insert the necessary error code for those.
Installation
npm install --save-dev @babel/plugin-transform-block-scoping
Usage
.babelrc
(Recommended)
Via .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
boolean
, 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:
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.
tdz
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:
i
let i;
If you need these errors you can tell Babel to try and find them by setting "tdz": true
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.