getFirstFunctionNameFromCallStack

getFirstFunctionNameFromCallStack.test.ts

import { getFirstFunctionNameFromCallStack } from './getFirstFunctionNameFromCallStack';

export const someSuperCoolAnonymousFunctionAssignedToAVariableIWantTheNameOf =
  () => {
    const methodName = getFirstFunctionNameFromCallStack();
    return { methodName };
  };

export const someSuperCoolNamedFunctionIWantTheNameOfWhichHappensToAlsoBeAssignedToAVariable =
  function someSuperCoolNamedFunctionIWantTheNameOf() {
    const methodName = getFirstFunctionNameFromCallStack();
    return { methodName };
  };

describe('getFirstMethodNameFromCallStack', () => {
  it('should return the name of the variable an anonymous function was assigned to, when run in an anonymous function', async () => {
    const { methodName } =
      someSuperCoolAnonymousFunctionAssignedToAVariableIWantTheNameOf();
    expect(methodName).toEqual(
      'someSuperCoolAnonymousFunctionAssignedToAVariableIWantTheNameOf',
    );
  });
  it('returns the declared name of the function, ignoring the name of the variable it was assigned to', () => {
    const { methodName } =
      someSuperCoolNamedFunctionIWantTheNameOfWhichHappensToAlsoBeAssignedToAVariable();
    expect(methodName).toEqual('someSuperCoolNamedFunctionIWantTheNameOf');
  });
});

getFirstFunctionNameFromCallStack.ts

/* eslint-disable no-restricted-syntax */

/**
 * returns the first name found in the call stack at runtime
 *
 * relevance
 * - find the name of the variable that an anonymous function was assigned to
 *
 * note
 * - uses Error.prepareStackTrace and Error.captureStackTrace to access the stack trace at runtime
 * - ⚠️ Error.captureStackTrace is an expensive operation; the results of this operation should be cached
 * - ⚠️ if the function you want the name of is defined in a jest test file, this method will always return `next`. (try defining your function in a different file and importing it instead)
 *
 * ref
 * - https://v8.dev/docs/stack-trace-api#customizing-stack-traces
 * - https://stackoverflow.com/a/21666875/3068233
 */
export const getFirstFunctionNameFromCallStack = (): string | null => {
  const obj = { stack: [] as any[] };
  const prepare = Error.prepareStackTrace;
  Error.prepareStackTrace = (_, stack) => stack;
  try {
    Error.captureStackTrace(obj);
    obj.stack.forEach((stackEntry, index) => {
      console.log({
        methodName: stackEntry.getMethodName(),
        functionName: stackEntry.getFunctionName(),
        index,
      });
    });
    return (
      obj.stack
        .find((stackEntry: any, index: number) => {
          // if its the first index, ignore it, since it will always show `getFirstMethodNameFromCallStack`
          if (index === 0) return false;

          // return the first stackEntry who's name is not null
          return stackEntry.getFunctionName() !== null;
        })
        ?.getFunctionName() ?? null // now grab the name, and default to null if no item was found
    );
  } finally {
    Error.prepareStackTrace = prepare; // ensure to _always_ reset the prepare function. otherwise, we would have borked all error stack tracing going forward 😬
  }
};