bind的使用场景
文章JavaScript容易混淆的地方提到了bind最简单的应用是用于绑定方法的运行时作用域。本文会再列举bind的几个应用场景。
声明：本文部分示例参考MDN文档
函数参数预设
下面这个方法可以将单个的参数转化为一个数组
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="javascript" cid="n6" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">function convertToArray(){
return Array.prototype.slice.call(arguments);
}</pre>
注：arguments是函数内部的一个类似函数的特殊对象，长得像这样{0:arg0, 1:arg1, length: 2}。slice方法若不传入参数会返回一个包含0到length-1数组。
现在我们想在这个方法返回的数组前预设一个元素，就可以这么做
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="javascript" cid="n9" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">var convertToArrayWithLeadingElement=convertToArray.bind(0);
var result=convertToArrayWithLeadingElement(null, 1,2,3);//[0,1,2,3]</pre>
改变setTimeout回调函数内部this的指向
在函数setTimeout中，this默认指向全局对象（node中是global，浏览器中是window），如果setTimeout用在对象的方法内部或者作为构造函数的原型方法，且需要使用对象的属性，我们必须为setTimeout的回调函数显性地绑定this对象。
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="javascript" cid="n12" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">var person={
name:"moya",
stutter: function(){
setTimeout(function(){
console.log(this.name);
}.bind(this), 1000)
}
}
person.stutter();//否则打印undefined</pre>
简化方法名称(shortcut)
以下面这个函数为例
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="javascript" cid="n15" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">function convertToArray(){
return Array.prototype.slice.call(arguments);
}</pre>
可以改为
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="javascript" cid="n17" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">var slice=Function.prototype.call.bind(Array.prototype.slice);//调用slice的时候会调用Array的原型方法slice
function convertToArray(){ return slice(arguments);}</pre>
吐槽一下：先了解即可，现在感觉并没有多简便。
最后来手动实现一下bind方法
bind方法在IE8（及以下）中不能用，可以通过下面的定义补救（我不太看得懂args.concat(slice.call(arguments))这一步的操作）
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind
👆请参见链接最下方Polyfill部分
参考文档
https://www.hacksparrow.com/javascript/convert-arguments-to-array.html
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind
