Is an anonymous function as a parameter a function declaration or a function expression?

Question

Assuming a function declaration is a statement where the function keyword is the first word of the statement, e.g.:

function() { console.log("foo") };

Assuming that a function expression is e.g. following:

for a named function

var func = function doSomething() { console.log("foo") };

for an anonymous function

var func = function() { console.log("foo") };

What is the case for the anonymous function, which is passed in as parameter in the following example:

for (let i = 0; i < 5; i++) {
    setTimeout(function() { console.log(i); }, 200); 
};

Is that a function declaration or is it a function expression since it is getting assigned to a parameter-variable of the setTimeout-method of WindowOrWorkerGlobalScope

Answer 1

Clearly a function expression

From MDN

Function expression

The function keyword can be used to define a function inside an expression.

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Syntax

let myFunction = function [name]([param1[, param2[, ..., paramN]]]) {
   statements
};

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A function expression is very similar to and has almost the same syntax as a function declaration (see function statement for details). The main difference between a function expression and a function declaration is the function name, which can be omitted in function expressions to create anonymous functions. A function expression can be used as an IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter about functions for more information.

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Function declaration

Syntax

function name([param[, param,[..., param]]]) {
   [statements]
}