getCaller.js

function getCaller() {
    const stack = getStack();

    stack.shift(); // getStack
    stack.shift(); // getCaller

    // Return caller's caller
    return stack;
}

function getStack() {
    // Save original Error.prepareStackTrace
    const origPrepareStackTrace = Error.prepareStackTrace;

    // Override with function that just returns `stack`
    Error.prepareStackTrace = (_, stack) => stack;

    // Create a new `Error`, which automatically gets `stack`
    // Evaluate `err.stack`, which calls our new `Error.prepareStackTrace`
    const { stack } = new Error();

    // Restore original `Error.prepareStackTrace`
    Error.prepareStackTrace = origPrepareStackTrace;

    // Remove superfluous function call on stack
    stack.shift(); // getStack --> Error

    return stack;
}

// EXAMPLE
// (() => getCaller())()[0].toString()
// "<anonymous>:1:20"