SOURCE

// apply
Function.prototype.myApply = function (thisArg, args = []) {
    thisArg = thisArg || window || global
    const fnSymol = Symbol('fn')
    thisArg[fnSymol] = this
    thisArg[fnSymol](...args)
    delete thisArg[fnSymol]
}

// call
Function.prototype.myCall = function (ctx, ...args) {
    ctx = ctx || window
    const fnSymol = Symbol('fn')
    ctx[fnSymol] = this
    ctx[fnSymol](...args)
    delete ctx[fnSymol]
}

// bind
Function.prototype.myBind = function (ctx, ...args) {
    ctx = ctx || window
    const fnSymol = Symbol('fn')
    ctx[fnSymol] = this
    return function(..._args) {
        args = args.concat(_args)
        ctx[fnSymol](...args)
        delete ctx[fnSymol]
    }
}

const obj = {
    a: 3
}

function f1() {
    console.log(this.a)
}

f1.myBind(obj)()