vue是三大流行前端框架，大家有时间可以研究一个vue的源码！
首先先我们来看看vue源码的开头,本人只把它当学习记录，如有错可以指出 谢谢：

(function (global, factory) {
    typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
    typeof define === 'function' && define.amd ? define(factory) :
    (global.Vue = factory());
}(this, (function () { 'use strict';
  //vue的源码都放在这里面，这是一个自执行function
 
})

源码代第14-336行基本是一些工具型的function,下面我来简单单说一下

var emptyObject = Object.freeze({});

// these helpers produces better vm code in JS engines due to their
// explicitness and function inlining
function isUndef (v) { //判断是否是undefined或null
  return v === undefined || v === null
}

function isDef (v) {//判断是否undefined与null
  return v !== undefined && v !== null
}

function isTrue (v) {
  return v === true
}

function isFalse (v) {
  return v === false
}

/**
 * Check if value is primitive
 */
function isPrimitive (value) { //判断类型 string number symbol boolean
  return (
    typeof value === 'string' ||
    typeof value === 'number' ||
    // $flow-disable-line
    typeof value === 'symbol' ||
    typeof value === 'boolean'
  )
}

/**
 * Quick object check - this is primarily used to tell
 * Objects from primitive values when we know the value
 * is a JSON-compliant type.
 */
function isObject (obj) { //判断是否为null并且是对象类型
  return obj !== null && typeof obj === 'object'
}

/**
 * Get the raw type string of a value e.g. [object Object]

在这里我说call方法和apply方法这是很常用的，这里是借用对象中的toString方法
call(obj,vaule);obj是一个对象，或方法，把obj的this指向obj,value是参数;就是给个toString方法传递参数
apply()与call几乎相同，不同的是call是一个参数，apply后面可以把很多个参数传过去apply(obj,val1,val2,val3…………);
 */
var _toString = Object.prototype.toString;

function toRawType (value) {//截取string的字符[object string]
  return _toString.call(value).slice(8, -1)
}

/**
 * Strict object type check. Only returns true
 * for plain JavaScript objects.
 */
function isPlainObject (obj) {//判断是否是对象
  return _toString.call(obj) === '[object Object]'
}

function isRegExp (v) {
  return _toString.call(v) === '[object RegExp]'//判断是否是正则对象
}

/**
 * Check if val is a valid array index.
 */
function isValidArrayIndex (val) {//判断是否为数组索引
  var n = parseFloat(String(val));//将值转为字符串后再转为小数类型
  return n >= 0 && Math.floor(n) === n && isFinite(val)
    //判断是否溢出了
}

/**
 * Convert a value to a string that is actually rendered.
 */
function toString (val) { //转为字符串
  return val == null
    ? ''
    : typeof val === 'object'
      ? JSON.stringify(val, null, 2)
      : String(val)
}

/**
 * Convert a input value to a number for persistence.
 * If the conversion fails, return original string.
 */
function toNumber (val) {
  var n = parseFloat(val);
  return isNaN(n) ? val : n
}

/**
 * Make a map and return a function for checking if a key
 * is in that map.
 */
function makeMap ( //判断json对象否存在该索引
  str,
  expectsLowerCase
) {
  var map = Object.create(null);//{}
  var list = str.split(',');//字符转数组
  for (var i = 0; i < list.length; i++) {
    map[list[i]] = true;
  }
  return expectsLowerCase
    ? function (val) { return map[val.toLowerCase()]; }
    : function (val) { return map[val]; }
}

/**
 * Check if a tag is a built-in tag.
 */
var isBuiltInTag = makeMap('slot,component', true);//不区分大小写

/**
 * Check if a attribute is a reserved attribute.
 */
var isReservedAttribute = makeMap('key,ref,slot,slot-scope,is');//区分大小写

/**
 * Remove an item from an array
 */
function remove (arr, item) {//移除数组一个元素
  if (arr.length) {
    var index = arr.indexOf(item);
    if (index > -1) {
      return arr.splice(index, 1)
    }
  }
}

/**
 * Check whether the object has the property.
 */
var hasOwnProperty = Object.prototype.hasOwnProperty;
function hasOwn (obj, key) {
  return hasOwnProperty.call(obj, key)  //查看obj是否存在key的属性
}

/**
 * Create a cached version of a pure function.
 */
function cached (fn) {//缓存
  var cache = Object.create(null);//{}
  return (function cachedFn (str) {
    var hit = cache[str];
    return hit || (cache[str] = fn(str))
  })
}

/**
 * Camelize a hyphen-delimited string.
 */
var camelizeRE = /-(\w)/g;
var camelize = cached(function (str) {
  return str.replace(camelizeRE, function (_, c) { return c ? c.toUpperCase() : ''; })
});

/**
 * Capitalize a string.
 */
var capitalize = cached(function (str) {
  return str.charAt(0).toUpperCase() + str.slice(1)
});

/**
 * Hyphenate a camelCase string.
 */
var hyphenateRE = /\B([A-Z])/g;
var hyphenate = cached(function (str) {
  return str.replace(hyphenateRE, '-$1').toLowerCase()
});

/**
 * Simple bind polyfill for environments that do not support it... e.g.
 * PhantomJS 1.x. Technically we don't need this anymore since native bind is
 * now more performant in most browsers, but removing it would be breaking for
 * code that was able to run in PhantomJS 1.x, so this must be kept for
 * backwards compatibility.
 */

/* istanbul ignore next */
function polyfillBind (fn, ctx) {
  function boundFn (a) {
    var l = arguments.length;
    return l
      ? l > 1
        ? fn.apply(ctx, arguments)
        : fn.call(ctx, a)
      : fn.call(ctx)
  }

  boundFn._length = fn.length;
  return boundFn
}

function nativeBind (fn, ctx) {
  return fn.bind(ctx)
}

var bind = Function.prototype.bind
  ? nativeBind
  : polyfillBind;

/**
 * Convert an Array-like object to a real Array.
 */
function toArray (list, start) {
  start = start || 0;
  var i = list.length - start;
  var ret = new Array(i);
  while (i--) {
    ret[i] = list[i + start];
  }
  return ret
}

/**
 * Mix properties into target object.
 */
function extend (to, _from) {
  for (var key in _from) {
    to[key] = _from[key];
  }
  return to
}

/**
 * Merge an Array of Objects into a single Object.
 */
function toObject (arr) {
  var res = {};
  for (var i = 0; i < arr.length; i++) {
    if (arr[i]) {
      extend(res, arr[i]);
    }
  }
  return res
}

/**
 * Perform no operation.
 * Stubbing args to make Flow happy without leaving useless transpiled code
 * with ...rest (https://flow.org/blog/2017/05/07/Strict-Function-Call-Arity/)
 */
function noop (a, b, c) {} //什么也没有

/**
 * Always return false.
 */
var no = function (a, b, c) { return false; };//返回false

/**
 * Return same value
 */
var identity = function (_) { return _; };返回_

/**
 * Generate a static keys string from compiler modules.
从编译器模块生成一个静态密钥字符串，这里我还不知道有什么用处
只知道大是将一个[{staticKeys:1254}, {staticKeys:1254},{staticKeys:1254}, {staticKeys:1254}] 这样的数组转成字符串
 */
function genStaticKeys (modules) {  
  return modules.reduce(function (keys, m) {
    return keys.concat(m.staticKeys || [])
  }, []).join(',')
}

/**
 * Check if two values are loosely equal - that is,
 * if they are plain objects, do they have the same shape?
 */
function looseEqual (a, b) {//判断两个值是否相等
  if (a === b) { return true }
  var isObjectA = isObject(a);
  var isObjectB = isObject(b);
  if (isObjectA && isObjectB) {
    try {
      var isArrayA = Array.isArray(a);
      var isArrayB = Array.isArray(b);
      if (isArrayA && isArrayB) {
        return a.length === b.length && a.every(function (e, i) {
          return looseEqual(e, b[i])
        })
      } else if (!isArrayA && !isArrayB) {
        var keysA = Object.keys(a);
        var keysB = Object.keys(b);
        return keysA.length === keysB.length && keysA.every(function (key) {
          return looseEqual(a[key], b[key])
        })
      } else {
        /* istanbul ignore next */
        return false
      }
    } catch (e) {
      /* istanbul ignore next */
      return false
    }
  } else if (!isObjectA && !isObjectB) {
    return String(a) === String(b)
  } else {
    return false
  }
}

function looseIndexOf (arr, val) {
  for (var i = 0; i < arr.length; i++) {
    if (looseEqual(arr[i], val)) { return i }
  }
  return -1
}

/**
 * Ensure a function is called only once.
只执行一次 function
 */
function once (fn) {
  var called = false;
  return function () {
    if (!called) {
      called = true;
      fn.apply(this, arguments);
    }
  }
}