Function.prototype.myBind.js

function sum(...numbers) {
    return  numbers.reduce((x, y) => x + y, this.initialValue)
}

Function.prototype.myBind = function(context, ...boundArgs) {
    return (...ownArgs) => {
        return this.apply(context, [...boundArgs, ...ownArgs])
    }
}


const context = {
    initialValue: 5
}

console.assert(sum.bind(context, 1)(2, 3) === sum.myBind(context, 1)(2, 3))