[pure-component] Parsing error: Identifier props has already been declared

[davidnormo]

I'm raising this issue for awareness rather than an ask to fix as I spent some time trying to but found issues in recast which need resolving first.

The Problem

When using the pure-component transform there is an error if a props variable has already been declared in the render method:

// ...inside MyComponent class
render() {
  const { className, ...props } = this.props

The above is transformed into:

function MyComponent(props) {
  const { className, ...props } = props // Parsing error

Investigation

• I dug in a bit and managed to resolve this trivial case + other variable declaration types (property, rest, member
  expressions) by renaming them to props2
• However when I looked at declarations in a nested block scope those would also be renamed when ideally they should remain unchanged

  function MyComponent(props) {
    if (true) {
      const props = 'bar' // scoped to the if statement therefore should not be transformed
    }

• This lead me to look into recast looking up variable bindings by scope and found that recast doesn't support block scoping (path.scope not accurate for const and let benjamn/ast-types#154)
• If this is sorted out then we can fix this issue more comprehensively in pure-component

Let me know if you want me to share my code so far

[davidnormo]

Update: I've opened a PR here benjamn/ast-types#455

[brayfe]

Running into this issue today using the pure-component transform to update React from 15.6 => 16.8. Is the solution here to wait until the above PR is merged? Any guidance would be highly appreciated!

[davidnormo]

@brayfe the PR is a bit stuck at the moment dealing with hoisting within scopes...

But my changes that fixed my trivial case are here: https://github.com/davidnormo/react-codemod/tree/pure-component-fix
Just beware that it may break some cases if the props variable is inside a block.