evaluation.js

import type NodePath from "./index";

// This file contains Babels metainterpreter that can evaluate static code.

const VALID_CALLEES = ["String", "Number", "Math"];
const INVALID_METHODS = ["random"];

/**
 * Walk the input `node` and statically evaluate if it's truthy.
 *
 * Returning `true` when we're sure that the expression will evaluate to a
 * truthy value, `false` if we're sure that it will evaluate to a falsy
 * value and `undefined` if we aren't sure. Because of this please do not
 * rely on coercion when using this method and check with === if it's false.
 *
 * For example do:
 *
 *   if (t.evaluateTruthy(node) === false) falsyLogic();
 *
 * **AND NOT**
 *
 *   if (!t.evaluateTruthy(node)) falsyLogic();
 *
 */

export function evaluateTruthy(): boolean {
  const res = this.evaluate();
  if (res.confident) return !!res.value;
}

/**
 * Walk the input `node` and statically evaluate it.
 *
 * Returns an object in the form `{ confident, value }`. `confident` indicates
 * whether or not we had to drop out of evaluating the expression because of
 * hitting an unknown node that we couldn't confidently find the value of.
 *
 * Example:
 *
 *   t.evaluate(parse("5 + 5")) // { confident: true, value: 10 }
 *   t.evaluate(parse("!true")) // { confident: true, value: false }
 *   t.evaluate(parse("foo + foo")) // { confident: false, value: undefined }
 *
 */

export function evaluate(): { confident: boolean; value: any } {
  let confident = true;
  let deoptPath: ?NodePath;
  const seen = new Map;

  function deopt(path) {
    if (!confident) return;
    deoptPath = path;
    confident = false;
  }

  let value = evaluate(this);
  if (!confident) value = undefined;
  return {
    confident: confident,
    deopt:     deoptPath,
    value:     value
  };

  // we wrap the _evaluate method so we can track `seen` nodes, we push an item
  // to the map before we actually evaluate it so we can deopt on self recursive
  // nodes such as:
  //
  //   var g = a ? 1 : 2,
  //       a = g * this.foo
  //
  function evaluate(path) {
    const { node } = path;

    if (seen.has(node)) {
      const existing = seen.get(node);
      if (existing.resolved) {
        return existing.value;
      } else {
        deopt(path);
        return;
      }
    } else {
      const item = { resolved: false };
      seen.set(node, item);

      const val = _evaluate(path);
      if (confident) {
        item.resolved = true;
        item.value = val;
      }
      return val;
    }
  }

  function _evaluate(path) {
    if (!confident) return;

    const { node } = path;

    if (path.isSequenceExpression()) {
      const exprs = path.get("expressions");
      return evaluate(exprs[exprs.length - 1]);
    }

    if (path.isStringLiteral() || path.isNumericLiteral() || path.isBooleanLiteral()) {
      return node.value;
    }

    if (path.isNullLiteral()) {
      return null;
    }

    if (path.isTemplateLiteral()) {
      let str = "";

      let i = 0;
      const exprs = path.get("expressions");

      for (const elem of (node.quasis: Array<Object>)) {
        // not confident, evaluated an expression we don't like
        if (!confident) break;

        // add on cooked element
        str += elem.value.cooked;

        // add on interpolated expression if it's present
        const expr = exprs[i++];
        if (expr) str += String(evaluate(expr));
      }

      if (!confident) return;
      return str;
    }

    if (path.isConditionalExpression()) {
      const testResult = evaluate(path.get("test"));
      if (!confident) return;
      if (testResult) {
        return evaluate(path.get("consequent"));
      } else {
        return evaluate(path.get("alternate"));
      }
    }

    if (path.isExpressionWrapper()) { // TypeCastExpression, ExpressionStatement etc
      return evaluate(path.get("expression"));
    }

    // "foo".length
    if (path.isMemberExpression() && !path.parentPath.isCallExpression({ callee: node })) {
      const property = path.get("property");
      const object = path.get("object");

      if (object.isLiteral() && property.isIdentifier()) {
        const value = object.node.value;
        const type = typeof value;
        if (type === "number" || type === "string") {
          return value[property.node.name];
        }
      }
    }

    if (path.isReferencedIdentifier()) {
      const binding = path.scope.getBinding(node.name);

      if (binding && binding.constantViolations.length > 0) {
        return deopt(binding.path);
      }

      if (binding && path.node.start < binding.path.node.end) {
        return deopt(binding.path);
      }

      if (binding && binding.hasValue) {
        return binding.value;
      } else {
        if (node.name === "undefined") {
          return binding ? deopt(binding.path) : undefined;
        } else if (node.name === "Infinity") {
          return binding ? deopt(binding.path) : Infinity;
        } else if (node.name === "NaN") {
          return binding ? deopt(binding.path) : NaN;
        }

        const resolved = path.resolve();
        if (resolved === path) {
          return deopt(path);
        } else {
          return evaluate(resolved);
        }
      }
    }

    if (path.isUnaryExpression({ prefix: true })) {
      if (node.operator === "void") {
        // we don't need to evaluate the argument to know what this will return
        return undefined;
      }

      const argument = path.get("argument");
      if (node.operator === "typeof" && (argument.isFunction() || argument.isClass())) {
        return "function";
      }

      const arg = evaluate(argument);
      if (!confident) return;
      switch (node.operator) {
        case "!": return !arg;
        case "+": return +arg;
        case "-": return -arg;
        case "~": return ~arg;
        case "typeof": return typeof arg;
      }
    }

    if (path.isArrayExpression()) {
      const arr = [];
      const elems: Array<NodePath> = path.get("elements");
      for (let elem of elems) {
        elem = elem.evaluate();

        if (elem.confident) {
          arr.push(elem.value);
        } else {
          return deopt(elem);
        }
      }
      return arr;
    }

    if (path.isObjectExpression()) {
      const obj = {};
      const props: Array<NodePath> = path.get("properties");
      for (const prop of props) {
        if (prop.isObjectMethod() || prop.isSpreadProperty()) {
          return deopt(prop);
        }
        const keyPath = prop.get("key");
        let key = keyPath;
        if (prop.node.computed) {
          key = key.evaluate();
          if (!key.confident) {
            return deopt(keyPath);
          }
          key = key.value;
        } else if (key.isIdentifier()) {
          key = key.node.name;
        } else {
          key = key.node.value;
        }
        const valuePath = prop.get("value");
        let value = valuePath.evaluate();
        if (!value.confident) {
          return deopt(valuePath);
        }
        value = value.value;
        obj[key] = value;
      }
      return obj;
    }

    if (path.isLogicalExpression()) {
      // If we are confident that one side of an && is false, or the left
      // side of an || is true, we can be confident about the entire expression
      const wasConfident = confident;
      const left = evaluate(path.get("left"));
      const leftConfident = confident;
      confident = wasConfident;
      const right = evaluate(path.get("right"));
      const rightConfident = confident;
      confident = leftConfident && rightConfident;

      switch (node.operator) {
        case "||":
          // TODO consider having a "truthy type" that doesn't bail on
          // left uncertainity but can still evaluate to truthy.
          if (left && leftConfident) {
            confident = true;
            return left;
          }

          if (!confident) return;

          return left || right;
        case "&&":
          if ((!left && leftConfident) || (!right && rightConfident)) {
            confident = true;
          }

          if (!confident) return;

          return left && right;
      }
    }

    if (path.isBinaryExpression()) {
      const left = evaluate(path.get("left"));
      if (!confident) return;
      const right = evaluate(path.get("right"));
      if (!confident) return;

      switch (node.operator) {
        case "-": return left - right;
        case "+": return left + right;
        case "/": return left / right;
        case "*": return left * right;
        case "%": return left % right;
        case "**": return left ** right;
        case "<": return left < right;
        case ">": return left > right;
        case "<=": return left <= right;
        case ">=": return left >= right;
        case "==": return left == right; // eslint-disable-line eqeqeq
        case "!=": return left != right;
        case "===": return left === right;
        case "!==": return left !== right;
        case "|": return left | right;
        case "&": return left & right;
        case "^": return left ^ right;
        case "<<": return left << right;
        case ">>": return left >> right;
        case ">>>": return left >>> right;
      }
    }

    if (path.isCallExpression()) {
      const callee = path.get("callee");
      let context;
      let func;

      // Number(1);
      if (
        callee.isIdentifier() && !path.scope.getBinding(callee.node.name, true) &&
        VALID_CALLEES.indexOf(callee.node.name) >= 0
      ) {
        func = global[node.callee.name];
      }

      if (callee.isMemberExpression()) {
        const object = callee.get("object");
        const property = callee.get("property");

        // Math.min(1, 2)
        if (
          object.isIdentifier() && property.isIdentifier() &&
          VALID_CALLEES.indexOf(object.node.name) >= 0 &&
          INVALID_METHODS.indexOf(property.node.name) < 0
        ) {
          context = global[object.node.name];
          func = context[property.node.name];
        }

        // "abc".charCodeAt(4)
        if (object.isLiteral() && property.isIdentifier()) {
          const type = typeof object.node.value;
          if (type === "string" || type === "number") {
            context = object.node.value;
            func = context[property.node.name];
          }
        }
      }

      if (func) {
        const args = path.get("arguments").map(evaluate);
        if (!confident) return;

        return func.apply(context, args);
      }
    }

    deopt(path);
  }
}