A string.

Because the stack property is non-standard, implementations differ about where it's installed.

• In Firefox, it's an accessor property on Error.prototype.
• In Chrome and Safari, it's a data property on each Error instance, with the descriptor:

Each JavaScript engine uses its own format for stack traces, but they are fairly consistent in their high-level structure. Every implementation uses a separate line in the stack to represent each function call. The call that directly caused the error is placed at the top, and the call that started the whole call chain is placed at the bottom. Below are some examples of stack traces:

function foo() {
  bar();
}

function bar() {
  baz();
}

function baz() {
  console.log(new Error().stack);
}

foo();

#### JavaScriptCore
[email protected]:10:24
[email protected]:6:6
[email protected]:2:6
global [email protected]:13:4

#### SpiderMonkey
[email protected]:10:15
[email protected]:6:3
[email protected]:2:3
@filename.js:13:1

#### V8
Error
    at baz (filename.js:10:15)
    at bar (filename.js:6:3)
    at foo (filename.js:2:3)
    at filename.js:13:1

Different engines set this value at different times. Most modern engines set it when the Error object is created. This means you can get the full call stack information within a function using the following:

function foo() {
  console.log(new Error().stack);
}

Without having to throw an error and then catch it.

In V8, the non-standard Error.captureStackTrace(), Error.stackTraceLimit, and Error.prepareStackTrace() APIs can be used to customize the stack trace. Read the Stack trace API in the V8 docs for more information.

Stack frames can be things other than explicit function calls, too. For example, event listeners, timeout jobs, and promise handlers all begin their own call chain. Source code within eval() and Function constructor calls also appear in the stack:

console.log(new Function("return new Error('Function failed')")().stack);
console.log("====");
console.log(eval("new Error('eval failed')").stack);

#### JavaScriptCore
anonymous@
global [email protected]:1:65
====
eval code@
eval@[native code]
global [email protected]:3:17

#### SpiderMonkey
[email protected] line 1 > Function:1:8
@filename.js:1:65

====
@filename.js line 3 > eval:1:1
@filename.js:3:13

#### V8
Error: Function failed
    at eval (eval at <anonymous> (filename.js:1:13), <anonymous>:1:8)
    at filename.js:1:65
====
Error: eval failed
    at eval (eval at <anonymous> (filename.js:3:13), <anonymous>:1:1)
    at filename.js:3:13

In Firefox, you can use the //# sourceURL directive to name an eval source. See the Firefox Debug eval sources docs and the Naming eval Scripts with the //# sourceURL Directive blog post for more details.

The following script demonstrates how to use the stack property to output a stack trace into your browser window. You can use this to check what your browser's stack structure looks like.

function trace() {
  throw new Error("trace() failed");
}
function b() {
  trace();
}
function a() {
  b(3, 4, "\n\n", undefined, {});
}
try {
  a("first call, firstarg");
} catch (e) {
  document.getElementById("output").textContent = e.stack;
}

Not part of any standard.

• TraceKit on GitHub
• stacktrace.js on GitHub
• Stack trace API in the V8 docs