Javascript get ref to Array.forEach

Question

Simple question - why

[1,2,3,4].forEach(console.log)

works fine,

let g = f => [1,2,3,4].forEach(f);
g(console.log);

works fine, but

let h = [1,2,3,4].forEach;
h(console.log)

throws Uncaught TypeError: Array.prototype.forEach called on null or undefined ?

Your first two examples are equivalent shorthand for [1,2,3].forEach(e => console.log(e)). The last example doesn't invoke Array.prototype.forEach. — msanford
– msanford, Commented Sep 24, 2019 at 14:58
Hi I think that forEach expect a callback as a parameter, In your first and second functions you are passing that, console.log, and f as a parameter — rcoro
– rcoro, Commented Sep 24, 2019 at 15:00

TKoL · Accepted Answer · 2019-09-24 14:59:08Z

3

let x = [1,2,3,4];
let h = x.forEach.bind(x);
h(console.log)

It's because the forEach function gets unbound when you assign it to a variable. That's why this code works

answered Sep 24, 2019 at 14:59

TKoL

13.9k4 gold badges49 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Nina Scholz · Accepted Answer · 2019-09-24 15:00:01Z

2

You are only taking the function forEach from the array, but you need to bind the array for iterating this array (Function#bind).

Only this step takes the array as this for the array method.

var array = [1, 2, 3, 4],
    h = Array.prototype.forEach.bind(array);

h(console.log);

answered Sep 24, 2019 at 15:00

Nina Scholz

388k26 gold badges367 silver badges417 bronze badges