no-immutable-reactive-statements.ts

import type { AST } from "svelte-eslint-parser"
import { createRule } from "../utils"
import type {
  Scope,
  Variable,
  Reference,
  Definition,
} from "@typescript-eslint/scope-manager"

export default createRule("no-immutable-reactive-statements", {
  meta: {
    docs: {
      description:
        "disallow reactive statements that don't reference reactive values.",
      category: "Best Practices",
      // TODO Switch to recommended in the major version.
      recommended: false,
    },
    schema: [],
    messages: {
      immutable:
        "This statement is not reactive because all variables referenced in the reactive statement are immutable.",
    },
    type: "suggestion",
  },
  create(context) {
    const scopeManager = context.getSourceCode().scopeManager
    const globalScope = scopeManager.globalScope
    const toplevelScope =
      globalScope?.childScopes.find((scope) => scope.type === "module") ||
      globalScope
    if (!globalScope || !toplevelScope) {
      return {}
    }

    const cacheMutableVariable = new WeakMap<Variable, boolean>()

    /**
     * Checks whether the given reference is a mutable variable or not.
     */
    function isMutableVariableReference(reference: Reference) {
      if (reference.identifier.name.startsWith("$")) {
        // It is reactive store reference.
        return true
      }
      if (!reference.resolved) {
        // Unknown variable
        return true
      }
      return isMutableVariable(reference.resolved)
    }

    /**
     * Checks whether the given variable is a mutable variable or not.
     */
    function isMutableVariable(variable: Variable) {
      const cache = cacheMutableVariable.get(variable)
      if (cache != null) {
        return cache
      }
      if (variable.defs.length === 0) {
        // Global variables are assumed to be immutable.
        return true
      }
      const isMutable = variable.defs.some((def) => {
        if (def.type === "Variable") {
          const parent = def.parent
          if (parent.kind === "const") {
            return false
          }
          const pp = parent.parent
          if (
            pp &&
            pp.type === "ExportNamedDeclaration" &&
            pp.declaration === parent
          ) {
            // Props
            return true
          }
          return hasWrite(variable)
        }
        if (def.type === "ImportBinding") {
          return false
        }

        if (def.node.type === "AssignmentExpression") {
          // Reactive values
          return true
        }
        return false
      })
      cacheMutableVariable.set(variable, isMutable)
      return isMutable
    }

    /** Checks whether the given variable has a write or reactive store reference or not. */
    function hasWrite(variable: Variable) {
      const defIds = variable.defs.map((def: Definition) => def.name)
      return variable.references.some(
        (reference) =>
          reference.isWrite() &&
          !defIds.some(
            (defId) =>
              defId.range[0] <= reference.identifier.range[0] &&
              reference.identifier.range[1] <= defId.range[1],
          ),
      )
    }

    /**
     * Iterates through references to top-level variables in the given range.
     */
    function* iterateRangeReferences(scope: Scope, range: [number, number]) {
      for (const variable of scope.variables) {
        for (const reference of variable.references) {
          if (
            range[0] <= reference.identifier.range[0] &&
            reference.identifier.range[1] <= range[1]
          ) {
            yield reference
          }
        }
      }
    }

    return {
      SvelteReactiveStatement(node: AST.SvelteReactiveStatement) {
        for (const reference of iterateRangeReferences(
          toplevelScope,
          node.range,
        )) {
          if (reference.isWriteOnly()) {
            continue
          }
          if (isMutableVariableReference(reference)) {
            return
          }
        }
        for (const through of toplevelScope.through.filter(
          (reference) =>
            node.range[0] <= reference.identifier.range[0] &&
            reference.identifier.range[1] <= node.range[1],
        )) {
          if (through.identifier.name.startsWith("$$")) {
            // Builtin `$$` vars
            return
          }
          if (through.resolved == null) {
            // Do not report if there are missing references.
            return
          }
        }

        context.report({
          node:
            node.body.type === "ExpressionStatement" &&
            node.body.expression.type === "AssignmentExpression" &&
            node.body.expression.operator === "="
              ? node.body.expression.right
              : node.body,
          messageId: "immutable",
        })
      },
    }
  },
})