MDN bind why concat arguments when calling apply
apply, bind, javascript
Solution
The `arguments` are in two different contexts there. Each time a function is invoked, among other things, an `arguments` object is set to all the arguments passed (if the `arguments` is accessed).
The first mention of `arguments` are the arguments to `bind()`, which will become initial arguments.
The second mention are the next set of arguments called on the bound proxy function. Because the `arguments` name would shadow its parent `arguments` (as well as the `this` context needing to be separated), they are stored under the name `aArgs`.
This allows for partial function application (sometimes referred to as currying).
var numberBases = parseInt.bind(null, "110");
console.log(numberBases(10));
console.log(numberBases(8));
console.log(numberBases(2));
jsFiddle.
Problem
MDN specifies a polyfill bind method for those browsers without a native bind method: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Function/bind This code has the following line: ``` aArgs.concat(Array.prototype.slice.call(arguments)) ``` Which is passed as the args to the apply method on the function: ``` fToBind.apply(this instanceof fNOP && oThis ? this : oThis, aArgs.concat(Array.prototype.slice.call(arguments))); ``` However, this line actually repeats the arguments, so that if I called the bind method as: ``` fnX.bind({value: 666}, 1, 2, 3) ``` the arguments passed to fnX are: ``` [1, 2, 3, Object, 1, 2, 3] ``` Run the following example and see the console output http://jsfiddle.net/dtbkq/ However the args reported by fnX are [1, 2, 3] which is correct. Can someone please explain why the args are duplicated when passed to the apply call but don't appear in the function arguments variable?