Can function expression be used in forEach method?

Question

Here's my example of code:

let arr = [18, 23, 44, 50];
let obj = {
    name: 'Kate',
};
arr.forEach(let func = function(value) {
    console.log(`${this.name} is ${value} years old`);
}, obj); //SyntaxError

So, it looks like we can use in forEach() method only function declaration and arrow function. Am I right or just missing something?

Check forEach syntax here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach — navnath, Sep 29 '21 at 16:06
I think you are trying to bind your obj like `function printUser(value) { console.log(this.name+' is '+value+' years old'); } arr.forEach(printUser.bind(obj));` — navnath, Sep 29 '21 at 16:15
`only function declaration` .. a function declared in expression context (eg. to the right of a `=` sign, inside `()` etc.) is a function expression. So **ONLY** function expressions can be used inside `forEach()` if you are writing a function literal (instead of a variable or function name) because the moment you put a function inside `()` it becomes an expression — slebetman, Sep 29 '21 at 16:25
@slebetman _"a function declared in expression context (eg. to the right of a = sign, **inside () etc.**) is a function expression..."_ I can't agree with the part about `()`. I've just checked [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function#syntax). There's only one variant of function expression syntax — Ivan, Sep 29 '21 at 16:41
May I ask, is there a reason why you're looking to do this? Or is it just out of interest? — OliverRadini, Sep 30 '21 at 08:00
[The `let func =` is not part of the function expression](https://stackoverflow.com/a/69395145/1048572). — Bergi, Nov 14 '21 at 07:42

score 0 · Answer 1 · answered Nov 14 '21 at 07:18

Using let, var, or const where a callback function is expected is the SyntaxError. You could drop let and the code will run fine or you could declare the function outside of forEach and call it within forEach as follows:

let func = function(value) {
    console.log(`${this.name} is ${value} years old`);
};
arr.forEach(func, obj);

In the code below, I show the original code you provided without the let keyword in the forEach call to show that it works. I also added the suggested function declaration and the corresponding forEach call.

let arr = [18, 23, 44, 50];
let obj = {
  name: 'Kate',
};

console.log('\tfunc');
arr.forEach(func = function(value) {
  console.log(`${this.name} is ${value} years old`);
}, obj);

console.log('\tfunc2');
let func2 = function(value) {
  console.log(`${this.name} is ${value} years old`);
};
arr.forEach(func2, obj);

score -1 · Answer 2 · answered Sep 29 '21 at 16:07

-1

You can use it without the assignment. The name (func) is optional.

let arr = [18, 23, 44, 50];
let obj = {
    name: 'Kate',
};
arr.forEach(function func(value) {
    console.log(`${this.name} is ${value} years old`);
}, obj);

answered Sep 29 '21 at 16:07

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Can function expression be used in forEach method?

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