三千文字，也没写好 Function.prototype.call

Function.prototype.call = function(context) {
    context = context || window;
    context["fn"] = this;
    let arg = [...arguments].slice(1); 
    context["fn"](...arg);
    delete context["fn"];
}
复制代码

Function.prototype.call = function (context) {
  context = (context == null || context == undefined) ? window : new  Object(context);
  context.fn = this;
  var arr = [];
  for (var i = 1; i < arguments.length; i++) {
    arr.push('arguments[' + i + ']');
  }
  var r = eval('context.fn(' + arr + ')');
  delete context.fn;
  return r;
}
复制代码

function.call(thisArg, arg1, arg2, ...)
复制代码

;void function(msg){
    console.log(msg)
}("你好啊");
复制代码

;(function(msg){
    console.log(msg)
})("你好啊");
复制代码

typeof self == 'object' && self.self === self
复制代码

typeof global == 'object' && global.global === global
复制代码

var getGlobal = function () {
  if (typeof self !== 'undefined') { return self; }
  if (typeof window !== 'undefined') { return window; }
  if (typeof global !== 'undefined') { return global; }
  throw new Error('unable to locate global object');
};
复制代码

var hasStrictMode = (function(){ 
 "use strict";
 return this == undefined;
}());
复制代码

var isStrict = (function(){
　　return this === undefined;
}());
复制代码

var r = eval('context.fn(' + arr + ')');
  delete context.fn;
复制代码

var context = {
  fn: "i am fn",
  msg: "i am msg"
}
log.call(context);  // i am msg
console.log("msg:", context.msg); // i am msg
console.log("fn:", context.fn); // fn: undedined
复制代码

var ctx = new Object(context);
var propertyName = "__fn__";
var originVal;
var hasOriginVal = ctx.hasOwnProperty(propertyName)
if(hasOriginVal){
    originVal = ctx[propertyName]
}
...... // 其他代码
if(hasOriginVal){
    ctx[propertyName] = originVal;
}
复制代码

When the call method is called on an object func with argument thisArg and optional arguments arg1, arg2 etc, the following steps are taken:
1. If IsCallable(func) is false, then throw a TypeError exception.
2. Let argList be an empty List.
3. If this method was called with more than one argument then in left to right 
order starting with arg1 append each argument as the last element of argList
4. Return the result of calling the [[Call]] internal method of func, providing 
thisArg as the this value and argList as the list of arguments.
The length property of the call method is 1.
NOTE The thisArg value is passed without modification as the this value. This is a 
change from Edition 3, where a undefined or null thisArg is replaced with the  
global object and ToObject is applied to all other values and that result is passed 
as the this value.
复制代码

var hasStrictMode = (function () {
    "use strict";
    return this == undefined;
}());
var isStrictMode = function () {
    return this === undefined;
};
var getGlobal = function () {
    if (typeof self !== 'undefined') { return self; }
    if (typeof window !== 'undefined') { return window; }
    if (typeof global !== 'undefined') { return global; }
    throw new Error('unable to locate global object');
};
function isFunction(fn){
    return typeof fn === "function";
}
function getContext(context) {
    var isStrict = isStrictMode();
    if (!hasStrictMode || (hasStrictMode && !isStrict)) {
        return (context === null || context === void 0) ? getGlobal() : Object(context);
    }
    // 严格模式下, 妥协方案
    return Object(context);
}
Function.prototype.call = function (context) {
    // 不可以被调用
    if (typeof this !== 'function') {
        throw new TypeError(this + ' is not a function');
    }
    // 获取上下文
    var ctx = getContext(context);
    // 更为稳妥的是创建唯一ID, 以及检查是否有重名
    var propertyName = "__fn__" + Math.random() + "_" + new Date().getTime();
    var originVal;
    var hasOriginVal = isFunction(ctx.hasOwnProperty) ? ctx.hasOwnProperty(propertyName) : false;
    if (hasOriginVal) {
        originVal = ctx[propertyName]
    }
    ctx[propertyName] = this;    
    // 采用string拼接
    var argStr = '';
    var len = arguments.length;
    for (var i = 1; i < len; i++) {
        argStr += (i === len - 1) ? 'arguments[' + i + ']' : 'arguments[' + i + '],'
    }
    var r = eval('ctx["' + propertyName + '"](' + argStr + ')');
    // 还原现场
    if (hasOriginVal) {
        ctx[propertyName] = originVal;
    } else {
        delete ctx[propertyName]
    }
    return r;
}
复制代码

[Report Only] Refused to evaluate a string as JavaScript because 'unsafe-eval' is not an   
allowed source of script in the following Content Security Policy directive: "script-src 
.........
复制代码

const sum = new Function('a', 'b', 'return a + b');
console.log(sum(2, 6));
// expected output: 8
复制代码

function generateFunctionCode(argsArrayLength){
    var code = 'return arguments[0][arguments[1]](';
    for(var i = 0; i < argsArrayLength; i++){
        if(i > 0){
            code += ',';
        }
        code += 'arguments[2][' + i + ']';
    }
    code += ')';
    // return arguments[0][arguments[1]](arg1, arg2, arg3...)
    return code;
}
复制代码

var hasStrictMode = (function () {
    "use strict";
    return this == undefined;
}());
var isStrictMode = function () {
    return this === undefined;
};
var getGlobal = function () {
    if (typeof self !== 'undefined') { return self; }
    if (typeof window !== 'undefined') { return window; }
    if (typeof global !== 'undefined') { return global; }
    throw new Error('unable to locate global object');
};
function isFunction(fn){
    return typeof fn === "function";
}
function getContext(context) {
    var isStrict = isStrictMode();
    if (!hasStrictMode || (hasStrictMode && !isStrict)) {
        return (context === null || context === void 0) ? getGlobal() : Object(context);
    }
    // 严格模式下, 妥协方案
    return Object(context);
}
function generateFunctionCode(argsLength){
    var code = 'return arguments[0][arguments[1]](';
    for(var i = 0; i < argsLength; i++){
        if(i > 0){
            code += ',';
        }
        code += 'arguments[2][' + i + ']';
    }
    code += ')';
    // return arguments[0][arguments[1]](arg1, arg2, arg3...)
    return code;
}
Function.prototype.call = function (context) {
    // 不可以被调用
    if (typeof this !== 'function') {
        throw new TypeError(this + ' is not a function');
    }
    // 获取上下文
    var ctx = getContext(context);
    // 更为稳妥的是创建唯一ID, 以及检查是否有重名
    var propertyName = "__fn__" + Math.random() + "_" + new Date().getTime();
    var originVal;
    var hasOriginVal = isFunction(ctx.hasOwnProperty) ? ctx.hasOwnProperty(propertyName) : false;
    if (hasOriginVal) {
        originVal = ctx[propertyName]
    }
    ctx[propertyName] = this;
    var argArr = [];
    var len = arguments.length;
    for (var i = 1; i < len; i++) {
        argArr[i - 1] = arguments[i];
    }
    var r = new Function(generateFunctionCode(argArr.length))(ctx, propertyName, argArr);
    // 还原现场
    if (hasOriginVal) {
        ctx[propertyName] = originVal;
    } else {
        delete ctx[propertyName]
    }
    return r;
}
复制代码

"use strict";
var context = {
    a: 1,
    log(msg){
        console.log("msg:", msg)
    }
};
Object.freeze(context);
context.fn = function(){
};
console.log(context.fn);
VM111 call:12 Uncaught TypeError: Cannot add property fn, object is not extensible
    at VM49 call:12
复制代码

"use strict";
var context = {
    a: 1,
    log(msg){
        console.log("msg:", msg)
    }
};
Object.freeze(context);
var ctx =  Object.create(context);
ctx.fn = function(){
}
console.log("fn:", typeof ctx.fn);  // fn: function
console.log("ctx.a", ctx.a);  // ctx.a 1
console.log("ctx.fn", ctx.fn); // ctx.fn ƒ (){}
复制代码