I thought before that function expression can be any function that is store in some variable. For example, this is function expression, because function is stored in func variable.
let func = function() {
console.log(5);
};And here's function expression, because callback function is stored in function's paremeter callback.
function func(callback) {
callback();
};
func(function() {
console.log(5);
});
//That's what I mean:
//let callback = function() {...}But... recently I've found out that this callback function can also be considered as function expression. Why? This function isn't stored in some variable.
let arr = [18, 50];
let obj = {
name: 'Karina',
};
arr.forEach(function func(value) {
console.log(`${this.name} is ${value} years old`);
}, obj);So, my question is...What exactly make function function expression?
I thought before that function expression can be any function that is store in some variable.
No. The variable to store to does not matter. The let func = …; is not part of the function expression, the function expression is only the part (expression) that comes in the middle.
let func = function() {
// ^^^^^^^^^^^^
console.log(5);
//^^^^^^^^^^^^^^^^^^
};
//^
This function isn't stored in some variable.
Actually there's no difference between your second and third snippet. You're passing the function constructed from the expression to a function, regardless how that function is declared (with parameters, with rest syntax, without parameters, or as a builtin like forEach).
But yes, you can have function expressions completely without variables, like in
!function() {…}();
// ^^^^^^^^^^^^^^
void function() {…};
// ^^^^^^^^^^^^^^
(function() {…})();
// ^^^^^^^^^^^^^^
Basically, function expressions are just the literal syntax (as in: object literal, string literal) for function objects.
I'm also learning about functional programming and found out that using a function as a value is indeed a function expression, as you said, by assigning an anonymous function to a variable. But I think the main difference relies on the function declaration being hoisted right? So context comes to play and all of that fun stuff :)