2 files changed, 234 insertions, 55 deletions
diff --git a/src/modules/helpers/eval.ts b/src/modules/helpers/eval.ts
new file mode 100644
index 00000000..033c5d35
--- /dev/null
+++ b/src/modules/helpers/eval.ts
@@ -0,0 +1,229 @@
+import { FakerError } from '../../errors/faker-error';
+import type { Faker } from '../../faker';
+
+const REGEX_DOT_OR_BRACKET = /\.|\(/;
+
+/**
+ * Resolves the given expression and returns its result. This method should only be used when using serialized expressions.
+ *
+ * This method is useful if you have to build a random string from a static, non-executable source
+ * (e.g. string coming from a developer, stored in a database or a file).
+ *
+ * It tries to resolve the expression on the given/default entrypoints:
+ *
+ * ```js
+ * const firstName = fakeEval('person.firstName', faker);
+ * const firstName2 = fakeEval('person.first_name', faker);
+ * ```
+ *
+ * Is equivalent to:
+ *
+ * ```js
+ * const firstName = faker.person.firstName();
+ * const firstName2 = faker.helpers.arrayElement(faker.rawDefinitions.person.first_name);
+ * ```
+ *
+ * You can provide parameters as well. At first, they will be parsed as json,
+ * and if that isn't possible, it will fall back to string:
+ *
+ * ```js
+ * const message = fakeEval('phone.number(+!# !## #### #####!)', faker);
+ * ```
+ *
+ * It is also possible to use multiple parameters (comma separated).
+ *
+ * ```js
+ * const pin = fakeEval('string.numeric(4, {"allowLeadingZeros": true})', faker);
+ * ```
+ *
+ * This method can resolve expressions with varying depths (dot separated parts).
+ *
+ * ```ts
+ * const airlineModule = fakeEval('airline', faker); // AirlineModule
+ * const airlineObject = fakeEval('airline.airline', faker); // { name: 'Etihad Airways', iataCode: 'EY' }
+ * const airlineCode = fakeEval('airline.airline.iataCode', faker); // 'EY'
+ * const airlineName = fakeEval('airline.airline().name', faker); // 'Etihad Airways'
+ * const airlineMethodName = fakeEval('airline.airline.name', faker); // 'bound airline'
+ * ```
+ *
+ * It is NOT possible to access any values not passed as entrypoints.
+ *
+ * This method will never return arrays, as it will pick a random element from them instead.
+ *
+ * @param expression The expression to evaluate on the entrypoints.
+ * @param faker The faker instance to resolve array elements.
+ * @param entrypoints The entrypoints to use when evaluating the expression.
+ *
+ * @see faker.helpers.fake() If you wish to have a string with multiple expressions.
+ *
+ * @example
+ * fakeEval('person.lastName', faker) // 'Barrows'
+ * fakeEval('helpers.arrayElement(["heads", "tails"])', faker) // 'tails'
+ * fakeEval('number.int(9999)', faker) // 4834
+ *
+ * @since 8.4.0
+ */
+export function fakeEval(
+  expression: string,
+  faker: Faker,
+  entrypoints: ReadonlyArray<unknown> = [faker, faker.rawDefinitions]
+): unknown {
+  if (expression.length === 0) {
+    throw new FakerError('Eval expression cannot be empty.');
+  }
+
+  if (entrypoints.length === 0) {
+    throw new FakerError('Eval entrypoints cannot be empty.');
+  }
+
+  let current = entrypoints;
+  let remaining = expression;
+  do {
+    let index: number;
+    if (remaining.startsWith('(')) {
+      [index, current] = evalProcessFunction(remaining, current);
+    } else {
+      [index, current] = evalProcessExpression(remaining, current);
+    }
+
+    remaining = remaining.substring(index);
+
+    // Remove garbage and resolve array values
+    current = current
+      .filter((value) => value != null)
+      .map((value): unknown =>
+        Array.isArray(value) ? faker.helpers.arrayElement(value) : value
+      );
+  } while (remaining.length > 0 && current.length > 0);
+
+  if (current.length === 0) {
+    throw new FakerError(`Cannot resolve expression '${expression}'`);
+  }
+
+  const value = current[0];
+  return typeof value === 'function' ? value() : value;
+}
+
+/**
+ * Evaluates a function call and returns the new read index and the mapped results.
+ *
+ * @param input The input string to parse.
+ * @param entrypoints The entrypoints to attempt the call on.
+ */
+function evalProcessFunction(
+  input: string,
+  entrypoints: ReadonlyArray<unknown>
+): [continueIndex: number, mapped: unknown[]] {
+  const [index, params] = findParams(input);
+  const nextChar = input[index + 1];
+  switch (nextChar) {
+    case '.':
+    case '(':
+    case undefined:
+      break; // valid
+    default:
+      throw new FakerError(
+        `Expected dot ('.'), open parenthesis ('('), or nothing after function call but got '${nextChar}'`
+      );
+  }
+
+  return [
+    index + (nextChar === '.' ? 2 : 1), // one for the closing bracket, one for the dot
+    entrypoints.map((entrypoint): unknown =>
+      // TODO @ST-DDT 2023-12-11: Replace in v9
+      // typeof entrypoint === 'function' ? entrypoint(...params) : undefined
+      typeof entrypoint === 'function' ? entrypoint(...params) : entrypoint
+    ),
+  ];
+}
+
+/**
+ * Tries to find the parameters of a function call.
+ *
+ * @param input The input string to parse.
+ */
+function findParams(input: string): [continueIndex: number, params: unknown[]] {
+  let index = input.indexOf(')', 1);
+  if (index === -1) {
+    throw new FakerError(`Missing closing parenthesis in '${input}'`);
+  }
+
+  while (index !== -1) {
+    const params = input.substring(1, index);
+    try {
+      // assuming that the params are valid JSON
+      return [index, JSON.parse(`[${params}]`) as unknown[]];
+    } catch {
+      if (!params.includes("'") && !params.includes('"')) {
+        try {
+          // assuming that the params are a single unquoted string
+          return [index, JSON.parse(`["${params}"]`) as unknown[]];
+        } catch {
+          // try again with the next index
+        }
+      }
+    }
+
+    index = input.indexOf(')', index + 1);
+  }
+
+  index = input.lastIndexOf(')');
+  const params = input.substring(1, index);
+  return [index, [params]];
+}
+
+/**
+ * Processes one expression part and returns the new read index and the mapped results.
+ *
+ * @param input The input string to parse.
+ * @param entrypoints The entrypoints to resolve on.
+ */
+function evalProcessExpression(
+  input: string,
+  entrypoints: ReadonlyArray<unknown>
+): [continueIndex: number, mapped: unknown[]] {
+  const result = REGEX_DOT_OR_BRACKET.exec(input);
+  const dotMatch = (result?.[0] ?? '') === '.';
+  const index = result?.index ?? input.length;
+  const key = input.substring(0, index);
+  if (key.length === 0) {
+    throw new FakerError(`Expression parts cannot be empty in '${input}'`);
+  }
+
+  const next = input[index + 1];
+  if (dotMatch && (next == null || next === '.' || next === '(')) {
+    throw new FakerError(`Found dot without property name in '${input}'`);
+  }
+
+  return [
+    index + (dotMatch ? 1 : 0),
+    entrypoints.map((entrypoint) => resolveProperty(entrypoint, key)),
+  ];
+}
+
+/**
+ * Resolves the given property on the given entrypoint.
+ *
+ * @param entrypoint The entrypoint to resolve the property on.
+ * @param key The property name to resolve.
+ */
+function resolveProperty(entrypoint: unknown, key: string): unknown {
+  switch (typeof entrypoint) {
+    case 'function': {
+      try {
+        entrypoint = entrypoint();
+      } catch {
+        return undefined;
+      }
+
+      return entrypoint?.[key as keyof typeof entrypoint];
+    }
+
+    case 'object': {
+      return entrypoint?.[key as keyof typeof entrypoint];
+    }
+
+    default:
+      return undefined;
+  }
+}
diff --git a/src/modules/helpers/index.ts b/src/modules/helpers/index.ts
index 6e993c84..edc00ea9 100644
--- a/src/modules/helpers/index.ts
+++ b/src/modules/helpers/index.ts
@@ -2,6 +2,7 @@ import type { Faker, SimpleFaker } from '../..';
 import { FakerError } from '../../errors/faker-error';
 import { deprecated } from '../../internal/deprecated';
 import { SimpleModuleBase } from '../../internal/module-base';
+import { fakeEval } from './eval';
 import { luhnCheckValue } from './luhn-check';
 import type { RecordKey } from './unique';
 import * as uniqueExec from './unique';
@@ -1460,66 +1461,15 @@ export class HelpersModule extends SimpleHelpersModule {
     // extract method name from between the {{ }} that we found
     // for example: {{person.firstName}}
     const token = pattern.substring(start + 2, end + 2);
-    let method = token.replace('}}', '').replace('{{', '');
-
-    // extract method parameters
-    const regExp = /\(([^)]*)\)/;
-    const matches = regExp.exec(method);
-    let parameters = '';
-    if (matches) {
-      method = method.replace(regExp, '');
-      parameters = matches[1];
-    }
-
-    // split the method into module and function
-    const parts = method.split('.');
-
-    let currentModuleOrMethod: unknown = this.faker;
-    let currentDefinitions: unknown = this.faker.rawDefinitions;
-
-    // Search for the requested method or definition
-    for (const part of parts) {
-      currentModuleOrMethod =
-        currentModuleOrMethod?.[part as keyof typeof currentModuleOrMethod];
-      currentDefinitions =
-        currentDefinitions?.[part as keyof typeof currentDefinitions];
-    }
-
-    // Make method executable
-    let fn: (...args: unknown[]) => unknown;
-    if (typeof currentModuleOrMethod === 'function') {
-      fn = currentModuleOrMethod as (args?: unknown) => unknown;
-    } else if (Array.isArray(currentDefinitions)) {
-      fn = () =>
-        this.faker.helpers.arrayElement(currentDefinitions as unknown[]);
-    } else {
-      throw new FakerError(`Invalid module method or definition: ${method}
-- faker.${method} is not a function
-- faker.definitions.${method} is not an array`);
-    }
-
-    // assign the function from the module.function namespace
-    fn = fn.bind(this);
-
-    // If parameters are populated here, they are always going to be of string type
-    // since we might actually be dealing with an object or array,
-    // we always attempt to the parse the incoming parameters into JSON
-    let params: unknown[];
-    // Note: we experience a small performance hit here due to JSON.parse try / catch
-    // If anyone actually needs to optimize this specific code path, please open a support issue on github
-    try {
-      params = JSON.parse(`[${parameters}]`);
-    } catch {
-      // since JSON.parse threw an error, assume parameters was actually a string
-      params = [parameters];
-    }
+    const method = token.replace('}}', '').replace('{{', '');
 
-    const result = String(fn(...params));
+    const result = fakeEval(method, this.faker);
+    const stringified = String(result);
 
     // Replace the found tag with the returned fake value
     // We cannot use string.replace here because the result might contain evaluated characters
     const res =
-      pattern.substring(0, start) + result + pattern.substring(end + 2);
+      pattern.substring(0, start) + stringified + pattern.substring(end + 2);
 
     // return the response recursively until we are done finding all tags
     return this.fake(res);