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@babel/preset-flow

This preset is recommended if you use Flow, a static type checker for JavaScript code. It includes the following plugins:

Example​

In

JavaScript
function foo(one: any, two: number, three?): string {}

Out

JavaScript
function foo(one, two, three) {}

Installation​

npm install --save-dev @babel/preset-flow

Usage​

babel.config.json
{
  "presets": ["@babel/preset-flow"]
}

Via CLI​

Shell
babel --presets @babel/preset-flow script.js

Via Node API​

JavaScript
require("@babel/core").transformSync("code", {
  presets: ["@babel/preset-flow"],
});

Options​

all​

boolean, defaults to false.

Flow will only parse Flow-specific features if a @flow pragma is present atop the file, or the all option is set inside the .flowconfig.

If you are using the all option in your Flow config, be sure to set this option to true to get matching behavior.

For example, without either of the above set, the following call expression with a type argument:

f<T>(e)

Would get parsed as a nested binary expression:

JavaScript
f < T > e;

allowDeclareFields​

boolean, defaults to false

Added in: v7.9.0

NOTE: This will be enabled by default in Babel 8

When enabled, type-only class fields are only removed if they are prefixed with the declare modifier:

JavaScript
class A {
  declare foo: string; // Removed
  bar: string; // Initialized to undefined
}

You can read more about configuring preset options here