Javascript Behavior when Function.prototype.bind is called serially [duplicate]

Question

I have a question about Function.prototype.bind.

function f() {
  return this;
}

const g=f.bind({foo:'foo'});
const h=g.bind({bar:'bar'});

console.log(h());    // expected {bar:'bar'} but get {foo:'foo'}

I was expecting the above snippet to produce either {foo,bar} or {bar} but instead I get {foo}.

Can you explain what is going on?

Calling bind() creates a new function that will ignore its this argument. No matter what you pass to that function, or .bind() it again, it's ignored. — Bergi
– Bergi, Commented Oct 23, 2022 at 21:40

Pointy · Accepted Answer · 2022-10-23 21:37:56Z

1

Using .bind() to create a function that "fixes" the value of this in the called function results in a function where this cannot be overridden.

What you get back from .bind() is a new function that will pass along arguments to the original target function, with a guarantee that in that called function the value of this will be what you ask for. Re-binding that function therefore does no good: you do get back a new function, but it calls a function (the first bound function) that explicitly ignores its own this value.

answered Oct 23, 2022 at 21:37

Pointy

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