Babel Notes

Just personal notes that I'm collecting as I learn more about how to write plugins for babel.

Making a property on an object
- t.property('init', t.identifier('foo'), t.literal('bar'))
- Not sure why, but the kind of the property is init, second parameter is key of the object, final one is the value
Value to node
- t.valueToNode()
Remove a node
- this.dangerouslyRemove()
A member expression would be something like Object.defineProperties
- t.memberExpression(t.identifier('Object'), t.identifier('defineProperties'))
An assignment expression takes in
- operator
- left
- right
- t.assignmentExpression('=', t.identifier('foo'), t.valueToNode(1))
A Call Expression is simply a function or method call expression

  t.callExpression(
    // Callee
    t.memberExpression(
      t.identifier('Object'),
      t.identifier('defineProperties')
    ),
    // Arguments
    [t.valueToNode(1)] 
  )

Evaluation

Path: babel/packages/babel/src/traversal/path/evaluation.js

Functions:

function evaluate(): { confident: boolean; value: any }
- Walk the input node and statically evaluate it.
function evaluateTruthy(): boolean
- Walk the input node and statically evaluate if it's truthy.

You actually have the ability to try and evaluate certain expressions when parsing the AST. For example, if you had a BinaryExpression, you could evaluate both the left and the right side.

visitor: {
  BinaryExpression() {
    const { value: left } = this.get('left').evaluate();
    const { value: right } = this.get('right').evaluate();
    
    // Fully evaluated BinaryExpression
    return t.valueToNode(left + right);
  }
}

In addition, you also have access to an evaluateTruthy function that will walk the input node and statically evaluate if it's truthy.

joshblack/babel-notes.md

Babel Notes

Evaluation