function getThis()( console.log(this) } getThis(); //returns Window {postMessage: ƒ, blur: ƒ, focus: ƒ, close: ƒ, frames: Window, ...} Listing 5-2Get the Current Context of a Function in the Global Scope 代码中调用函数的地方称为执行上下文。执行上下文决定了this 的值。注...
i// Evaluates to the value of the variable i.sum// Evaluates to the value of the variable sum.undefined// The value of the "undefined" property of the global object 当程序中出现任何标识符时,JavaScript 假定它是一个变量、常量或全局对象的属性,并查找其值。如果不存在具有该名称的变量,则尝试评...
let count = 0; // Define a variable count++; // Increment the variable count--; // Decrement the variable count += 2; // Add 2: same as count = count + 2; count *= 3; // Multiply by 3: same as count = count * 3; count // => 6: variable names are expressions, too. ...
the set of data that has nothing to do with the display of itself) to grouping the data that’s appropriate for our “view” of that model (in this case, the current person to view). We call this variable the “view model” after a famous and useful technique...
鉴于它的影响,毫不奇怪 JavaScript 可以实现一种混合了函数式编程(高阶函数;内置的map,reduce等)和面向对象编程(对象,继承)的编程风格。 语法 本节解释了 JavaScript 的基本语法原则。 语法概述 一些语法的例子: // Two slashes start single-line commentsvarx;// declaring a variablex=3+y;// assigning a ...
QuickJS 是在 MIT 许可下发的一个轻量 js 引擎包含 js 的编译器和解释器,支持最新 TC39 的 ECMA-262 标准。QuickJS 和其它 js 引擎的性能对比,可以参看 QuickJS 的 benchmark 对比结果页,从结果看,JerryScript 内存和体积小于 QuickJS,但各项性能均低于 QuickJS,Hermes 体积和内存大于 QuickJS,性能和 QuickJS 差...
ES6 added 2 new global number methods: isFinite() isNaN() The isFinite() Method The globalisFinite()method returnsfalseif the argument isInfinityorNaN. Otherwise it returnstrue: Example isFinite(10/0);// returns false isFinite(10/1);// returns true ...
It's a JavaScript convention to use camel case for variable names with more than one word; for example, the variable className. Console message As a web developer, you can create hidden messages that aren't visible on your webpage, but that you can read in the Developer Tools in the Con...
Prevents accidental globals.Without strict mode, assigning a value to an undeclared variable automatically creates a global variable with that name. This is one of the most common JavaScript errors. In strict mode, attempting to do so throws an error. ...
包含在元素内部的 JavaScript 代码将被从上至下依次解释。就拿前面这个例子来说,解释器会解释一个函数的定义,然后将该定义保存在自己的环境当中。在解释器对元素内部的所有代码求值完毕以前,页面中的其余内容都不会被浏览器加载或显示。 如果要通过元素来包含外部 JavaScript 文件,那么 src 属性就是必需的。这个属性的...