实现call()

Function.prototype.call2 = function (context) {

    var context = context || window
    context.fn = this

    var args = []
    for (var i = 1, len = arguments.length; i < len; i++) {
        args.push('arguments[' + i + ']');
    }

    // 这里 args 会自动调用 Array.toString() 这个方法。
    var result = eval('context.fn(' + args + ')')

    delete context.fn
    return result;
}

ES6 写法

Function.prototype.call3 = function (context) {
    let target = context || window;
    target.fn = this

    let args = []
    for (let i = 1; i < arguments.length; i++) {
        args.push(arguments[i])
    }

    let result = target.fn(...args)

    delete target.fn

    return result
}
fn.call3(obj, '张三', '18')