OptionalChainingNullishTransformer.ts

import NameManager from "../NameManager";
import {TokenType as tt} from "../parser/tokenizer/types";
import TokenProcessor from "../TokenProcessor";
import Transformer from "./Transformer";

/**
 * Transformer supporting the optional chaining and nullish coalescing operators.
 *
 * Tech plan here:
 * https://github.com/alangpierce/sucrase/wiki/Sucrase-Optional-Chaining-and-Nullish-Coalescing-Technical-Plan
 *
 * The prefix and suffix code snippets are handled by TokenProcessor, and this transformer handles
 * the operators themselves.
 */
export default class OptionalChainingNullishTransformer extends Transformer {
  constructor(readonly tokens: TokenProcessor, readonly nameManager: NameManager) {
    super();
  }

  process(): boolean {
    if (this.tokens.matches1(tt.nullishCoalescing)) {
      this.tokens.replaceTokenTrimmingLeftWhitespace(", () =>");
      return true;
    }
    if (this.tokens.matches1(tt._delete)) {
      const nextToken = this.tokens.tokenAtRelativeIndex(1);
      if (nextToken.isOptionalChainStart) {
        this.tokens.removeInitialToken();
        return true;
      }
    }
    const token = this.tokens.currentToken();
    const chainStart = token.subscriptStartIndex;
    if (chainStart != null && this.tokens.tokens[chainStart].isOptionalChainStart) {
      const param = this.nameManager.claimFreeName("_");
      let arrowStartSnippet;
      if (
        chainStart > 0 &&
        this.tokens.matches1AtIndex(chainStart - 1, tt._delete) &&
        this.isLastSubscriptInChain()
      ) {
        // Delete operations are special: we already removed the delete keyword, and to still
        // perform a delete, we need to insert a delete in the very last part of the chain, which
        // in correct code will always be a property access.
        arrowStartSnippet = `${param} => delete ${param}`;
      } else {
        arrowStartSnippet = `${param} => ${param}`;
      }
      if (
        this.tokens.matches2(tt.questionDot, tt.parenL) ||
        this.tokens.matches2(tt.questionDot, tt.lessThan)
      ) {
        this.tokens.replaceTokenTrimmingLeftWhitespace(`, 'optionalCall', ${arrowStartSnippet}`);
      } else if (this.tokens.matches2(tt.questionDot, tt.bracketL)) {
        this.tokens.replaceTokenTrimmingLeftWhitespace(`, 'optionalAccess', ${arrowStartSnippet}`);
      } else if (this.tokens.matches1(tt.questionDot)) {
        this.tokens.replaceTokenTrimmingLeftWhitespace(`, 'optionalAccess', ${arrowStartSnippet}.`);
      } else if (this.tokens.matches1(tt.dot)) {
        this.tokens.replaceTokenTrimmingLeftWhitespace(`, 'access', ${arrowStartSnippet}.`);
      } else if (this.tokens.matches1(tt.bracketL)) {
        this.tokens.replaceTokenTrimmingLeftWhitespace(`, 'access', ${arrowStartSnippet}[`);
      } else if (this.tokens.matches1(tt.parenL)) {
        this.tokens.replaceTokenTrimmingLeftWhitespace(`, 'call', ${arrowStartSnippet}(`);
      } else {
        throw new Error("Unexpected subscript operator in optional chain.");
      }
      return true;
    }
    return false;
  }

  /**
   * Determine if the current token is the last of its chain, so that we know whether it's eligible
   * to have a delete op inserted.
   *
   * We can do this by walking forward until we determine one way or another. Each
   * isOptionalChainStart token must be paired with exactly one isOptionalChainEnd token after it in
   * a nesting way, so we can track depth and walk to the end of the chain (the point where the
   * depth goes negative) and see if any other subscript token is after us in the chain.
   */
  isLastSubscriptInChain(): boolean {
    let depth = 0;
    for (let i = this.tokens.currentIndex() + 1; ; i++) {
      if (i >= this.tokens.tokens.length) {
        throw new Error("Reached the end of the code while finding the end of the access chain.");
      }
      if (this.tokens.tokens[i].isOptionalChainStart) {
        depth++;
      } else if (this.tokens.tokens[i].isOptionalChainEnd) {
        depth--;
      }
      if (depth < 0) {
        return true;
      }

      // This subscript token is a later one in the same chain.
      if (depth === 0 && this.tokens.tokens[i].subscriptStartIndex != null) {
        return false;
      }
    }
  }
}