Hướng dẫn javascript function caller

Description

If the function f was invoked by the top-level code, the value of f.caller is null; otherwise it's the function that called f. If the function that called f is a strict mode function, the value of f.caller is also null.

In strict mode, accessing caller of a function throws an error. This is to prevent a function from being able to "walk the stack", which both poses security risks and severely limits the possibility of optimizations like inlining and tail-call optimization. For more explanation, you can read the rationale for the deprecation of arguments.callee.

Note that the only behavior specified by the ECMAScript specification is that Function.prototype has an initial caller accessor that unconditionally throws a TypeError for any get or set request (known as a "poison pill accessor"), and that implementations are not allowed to change this semantic for any function except non-strict plain functions, in which case it must not have the value of a strict mode function. The actual behavior of the caller property, if it's anything other than throwing an error, is implementation-defined. For example, Chrome defines it as an own data property, while Firefox and Safari extend the initial poison-pill Function.prototype.caller accessor to specially handle this values that are non-strict functions.

(function f() {
  if (Object.hasOwn(f, "caller")) {
    console.log(
      "caller is an own property with descriptor",
      Object.getOwnPropertyDescriptor(f, "caller"),
    );
  } else {
    console.log(
      "f doesn't have an own property named caller. Trying to get f.[[Prototype]].caller",
    );
    console.log(
      Object.getOwnPropertyDescriptor(
        Object.getPrototypeOf(f),
        "caller",
      ).get.call(f),
    );
  }
})();

// In Chrome:
// caller is an own property with descriptor {value: null, writable: false, enumerable: false, configurable: false}

// In Firefox:
// f doesn't have an own property named caller. Trying to get f.[[Prototype]].caller
// null

This property replaces the obsolete arguments.caller property of the arguments object.

The special property __caller__, which returned the activation object of the caller thus allowing to reconstruct the stack, was removed for security reasons.

Examples

Checking the value of a function's caller property

The following code checks the value a function's caller property.

function myFunc() {
  if (myFunc.caller === null) {
    return "The function was called from the top!";
  } else {
    return `This function's caller was ${myFunc.caller}`;
  }
}

Reconstructing the stack and recursion

Note that in case of recursion, you can't reconstruct the call stack using this property. Consider:

function f(n) {
  g(n - 1);
}
function g(n) {
  if (n > 0) {
    f(n);
  } else {
    stop();
  }
}
f(2);

At the moment stop() is called the call stack will be:

f(2) -> g(1) -> f(1) -> g(0) -> stop()

The following is true:

stop.caller === g && f.caller === g && g.caller === f;

so if you tried to get the stack trace in the stop() function like this:

let f = stop;
let stack = "Stack trace:";
while (f) {
  stack += `\n${f.name}`;
  f = f.caller;
}

the loop would never stop.

Strict mode caller

If the caller is a strict mode function, the value of caller is null.

function callerFunc() {
  calleeFunc();
}

function strictCallerFunc() {
  "use strict";
  calleeFunc();
}

function calleeFunc() {
  console.log(calleeFunc.caller);
}

(function () {
  callerFunc();
})();
// Logs [Function: callerFunc]

(function () {
  strictCallerFunc();
})();
// Logs null

Specifications

Not part of any standard.

See also